Kant and Jakob Facing Mendelssohn’s New Proof for the Existence of God: A Case of Misunderstanding

Rogelio Rovira
University of Madrid, España

Revista de Estudios Kantianos. Publicación internacional de la SEKLE

Universitat de València, España

ISSN-e: 2445-0669

Periodicity: Semestral

vol. 8, no. 2, 2024

Received: 16 November 2022

Accepted: 03 August 2023

Los autores conservan los derechos de autor y garantizan a la revista el derecho de ser la primera publicación del trabajo al igual que licenciado bajo una Creative Commons Attribution License que permite a otros compartir el trabajo con un reconocimiento de la autoría del trabajo y la publicación inicial en esta revista.

Abstract: After an overview of Mendelssohn’s catalogue of proofs for the existence of God, a detailed analysis of his new and original proof from the imperfectness of our self-knowledge, set out in Morning hours, is offered. Kant’s and his disciple Jakob’s criticisms are then stated. The former refers very briefly to the Mendelssohnian proof on two occasions. The latter, instead, scrutinises it thoroughly in his Examination of Mendelssohn’s Morning hours. The main result of this confrontation is that Kant and Jakob fail to adequately assess the peculiarity and novelty of Mendelssohn’s proof since both thinkers interpret the proof as being based not so much on the imperfection of our self-knowledge as on the demands of our reason in thinking the possible and the actual.

Keywords: Moses Mendelssohn, Ludwig Heinrich Jakob, Imperfection of self-knowledge, Possibility, Actuality.

Resumen: Tras una revisión del catálogo de pruebas de Mendelssohn a favor de la existencia de Dios, se ofrece un análisis detallado de la nueva y original prueba que expone en Horas matinales y que construye a partir de la imperfección del conocimiento de nosotros mismos. A continuación, se exponen las críticas de Kant y de su discípulo Jakob. El primero se refiere muy brevemente a la prueba mendelssohniana en dos ocasiones. El segundo, en cambio, la analiza a fondo en su Examen de las horas matinales de Mendelssohn. El principal resultado de esta confrontación es que Kant y Jakob no valoran adecuadamente la peculiaridad y novedad de la prueba de Mendelssohn, ya que ambos pensadores interpretan que la prueba no se basa tanto en la imperfección del conocimiento de nosotros mismos cuanto en las exigencias de nuestra razón al pensar lo posible y lo real.

Palabras clave: Moses Mendelssohn, Ludwig Heinrich Jakob, Imperfección del conocimiento de sí mismo, Posibilidad, Realidad.

1. Introduction

Few philosophers have explored with more rigour and originality than Mendelssohn the two possible ways of proving the existence of God, “the extraordinary and nonpareil path” from the possibility of a necessary being to its actuality, and the “easiest” path founded on “an experiential proposition” (Mendelssohn, 1764a, pp. 294, 299; 1997, pp. 275, 281). The accuracy, depth, and novelty of Mendelssohn’s efforts in natural theology were even eloquently testified at the time by the praise of thinkers who were not exactly supporters of his philosophy, as was the case with Kant’s early disciples.[2] It is worth considering briefly the Mendelssohnian catalogue of theistic proofs.

In the Wolffian tradition two a priori arguments are mainly considered, the Leibnizian proof from eternal truths, and the argument that Kant later called ‘ontological’. In his review of Lambert’s New Organon, Mendelssohn had occasion to express his doubts concerning the first proof, since “the inference from the possibility of representations to the existence of a representing being still seems to pose some difficulty” (1766, p. 45). Only the ‘ontological’ or Cartesian proof is regarded by Mendelssohn as fully cogent. He devoted long meditations to this proof both in his 1764 essay On evidence in metaphysical sciences (1764a, pp. 300-301; 1997, pp. 281-283) and in a short and unpublished paper, “The existence of God proved a priori” (1778).[3] In his Morning hours Mendelssohn (1785, pp. 148-157; 2011, pp. 109-115) again defends the ontological proof by providing an accurate and complete refutation of Kant’s criticisms, although without mentioning the philosopher’s name (Rovira, 2017).

Mendelssohn distinguishes two kinds of a posteriori proofs: those relying on the testimony of the external senses and those based upon the experiences of the inner sense. This classification does not coincide with Kant’s distinction between the ‘physico-theological’ and the ‘cosmological’ proof. Kant’s classification is made indeed with a different criterion, that of the “determinate” or “indeterminate” experience, which ultimately amounts to the distinction between matter and form of experience (KrV, A590/B618; 1998, p. 563), in such a way that for Kant the cosmological proof in all its possible forms is ultimately a transcendental proof, i.e. a non-empirical argument (KrV, A614/B643; 1998, p. 575).[4]

Mendelssohn divides the proofs relying on an empirical proposition concerning objects of external perception into three main classes: those based upon the beauty and order either in the visible parts of the world or in the laws of motion; those based upon the purposes in nature; and those based upon the contingency of the world. The first two types are briefly discussed in the 1764 prize essay. Mendelssohn’s judgement is blunt: these proofs lack “a great deal for demonstrative certainty”, although they all produce “practical conviction” (1764a, pp. 312-313; 1997, pp. 292-293). In his Morning hours Mendelssohn discusses two arguments based upon the contingency of the world. The first proof relies on the impossibility of the world existing without beginning or end in order to infer the existence of a necessary and eternal being. Mendelssohn takes up the criticisms of ‘many philosophers’ which clearly point out the unsatisfactory nature of this argument (1785, pp. 91-94; 2011, pp. 67-68). However, he considers himself justified to infer “the existence of the necessary and immutable from the existence of the contingent and mutable”, without needing to presuppose the impossibility of a beginningless world (1785, p. 95; 2011, p. 69). In his formulation of this second proof from contingency, Mendelssohn draws on Leibniz’s thought in an attempt to show that the dependency of the contingent being’s existence upon the necessary being ultimately relies on “the necessary being’s approval and free choice” (1785, p. 97; 2011, p. 70).

In general, Mendelssohn considers the proofs for God’s existence based upon the information of outer senses less compelling than those relying on the testimony of the inner sense, because “what the senses perceive of external things is dubious” (1764a, p. 294; 1997, p. 275). For this reason, both in the essay On evidence and in the Morning hours Mendelssohn proposes a formulation of the proof from contingency that rests on “the immediate feeling of my own existence”, which is “beyond all doubt” (1785, p. 141; 2011, p. 103). In this proof, one infers “from the undeniable intuitive proposition ‘I think’ to my actuality and from this to the actuality of a necessary being, by means of the principle of sufficient reason” (Mendelssohn, 1764a, p. 299; 1997, p. 281).

Despite his professed ignorance of Kant’s criticism of rational theology–if we are to believe his words in the prologue to his Morning hours–Mendelssohn could not have ignored two facts. The first one is that, in the Critique of pure reason, Kant summarises the so-called “cosmological argument” for the existence of God in much the same way as Mendelsohn himself, “If something exists, then an absolutely necessary being also has to exist. Now I myself, at least, exist; therefore, an absolutely necessary being exists” (KrV, A604/B633; Kant, 1998, p. 570). The second fact is that Kant’s main objection against this proof is that it does not by itself prove that an absolutely necessary being is also the most real being, i.e. God. In order to be cogent, the proof would have to presuppose that from the concept of an ens realissimum the concept of an ens neccesarium may be inferred –“a proposition the ontological proof asserted” (KrV, A606-607/B634-635; 1998, p. 571). Therefore, Kant claims, “it is really only the ontological proof from mere concepts that contains all the force of proof in the so-called cosmological proof” (KrV, A607/B635; 1998, p. 571). It is then reasonable to think, as has been suggested (Guyer, 2020, pp. 134-135), that in order to refute this criticism by Kant, Mendelssohn attempted in his last published work another formulation of the proof from contingency. The new formulation, based again on the internal experience, represents, according to Mendelssohn, an original and novel argument for the existence of God never before proposed, “I will attempt to conduct this proof in another way as well, in a way that, as far as I know, no philosopher has touched on” (1785, p. 141; 2011, p. 103).

The new proof takes as its starting point “the imperfection of our self-knowledge” in order to infer the existence of “an infinite intellect” which can only be that of God (Mendelssohn, 1785, pp. 146-147; 2011, p. 107). It relies on certain reflections concerning the possible and the actual. Not surprisingly, the proof aroused the interest of Kant, who refers to it on two occasions, and that of his early disciple Ludwig Heinrich Jakob, who discusses it in a lengthy chapter of his Examination of Mendelssohn’s Morning hours.

What does Mendelssohn’s new proof consist of? How did Kant and Jakob understand this proof and what criticisms do they propose against it? Are these objections relevant? In what follows I will attempt to show that neither Kant nor Jakob seem to be able to properly appreciate the peculiarity and novelty of Mendelssohn’s proof, because both thinkers neglect its starting point and focus their objections on the rational demands to think the possible and the actual.

2. Mendelssohn’s new proof for the existence of God from the imperfectness of our self-knowledge

After clarifying the correlative concepts of necessity and contingency, independence and dependence, Mendelssohn devotes the penultimate lecture of his Morning hours, the sixteenth, to his “Attempt at a new proof for the existence of God on the basis of the incompleteness of self-knowledge”, as the title of the lecture reads. Mendelssohn calls it “a new scientific proof for God’s existence” (1785, p. 147; 2011, p. 107). These words, however, do not contradict the modesty of the sage, who asks his listeners to remind him whenever, “out of predilection for my thoughts, I might permit myself some false step” (Mendelssohn, 1785, p. 141; 2011, p. 103).

Precisely in order to account for the steps of his argumentative process, Mendelssohn presents the proof in a pedagogical way in two phases. He first gives an overview of the complete demonstrative structure of the proof. Then he provides a more detailed justification of its nervus probandi, i.e. of the propositions which, if granted, the conclusion inevitably follows.

The complete demonstrative structure of Mendelssohn’s proof can be schematised, using in all cases his own words, in the following way:

[Starting point: indubitable perception of the inner sense]

1. “I do not know everything that pertains to my own existence”. In other words, “I am not merely what I distinctly know of myself or, what amounts to the same, there is more to my existence than I might consciously observe of myself” (1785, p. 141; 2011, p. 103).

[Truths involved in the starting point]

2. Possibilities and actualities that I do not know belong to my make-up [Beschaffenheit] as possible and actual predicates. “In fact it would not be possible for either our body or our soul to be on hand if they were merely what we distinctly observe of them” (1785, p. 142; 2011, p. 103).

3. Thus, between my knowledge of my own constitution (the concept) and my own constitution (the thing) there is no perfect harmony. “In a word, between concept and thing, if I look merely at my knowledge of myself, the most perfect harmony is not to be found” (1785, p. 146; 2011, p. 106).

[‘Existential moment’ of the proof]

4. But between a thing and its concept there has to be perfect harmony. “This agreement between a thing and [its] concept knows no exception” (1785, p. 145, 2011, p. 106).

5. [Nervus probandi] The reasons for the agreement between thing and concept rely on these two truths:

a. “Everything possible must be thought to be possible by some thinking being”.

b. “Everything actual must be thought to be actual by some thinking being” (1785, p. 142; 2011, p. 104).

6. “[A]n entity must be on hand (vorhanden seyn) which represents to itself in the most distinct, purest, and most thoroughgoing manner everything that pertains to my existence” (1785, p. 142; 2011, p. 104).

[‘Essential moment’ of the proof]

7. “No truth can be thought to be possible by contingent beings with the highest degree of knowledge, no actuality can be thought to be actual [by them] in the most perfect way” (1785, p. 143; 2011, p. 104). In other words, “I cannot dispute the fact that a limited being, indeed, that the sum total of all limited beings – whether finite or infinite in number – does not suffice to know my constitutive features in a way that harmonizes with the thing” (1785, p. 146; 2011, p. 106).

8. [Conclusio] “There must, therefore, be one thinking being, one intellect that thinks in the most perfect way the sum total of all possibilities as possible and the sum total of all actualities as actual” (1785, p. 143; 2011, p. 104). Briefly, “There is an infinite intellect” (1785, p. 146; 2011, p. 107).

Both the starting point and the truths involved in it (steps 1 to 3) are regarded by Mendelssohn to be immediately known and subjectively certain. As perceptions of the inner sense, it is not possible to be deceived about them. The imperfection of my self-knowledge is then as evident and undeniable as the consciousness of my own existence. As Leo Strauss (2012, p. 183) points out in one of his editorial annotations to Morning hours, “Mendelssohn starts from self-knowledge because, and only because, he wants to secure the proof against the Idealist doubt of the existence of the corporeal world”.

The ‘existential moment’ of the proof (steps 4 to 6) rests on a twofold assertion, which can be characterised as the true demonstrative core (the nervus probandi) of the argument (step 5), namely, that everything possible must be thought of as possible and that everything actual must be thought of as actual. These propositions are substantiated by Mendelssohn in two ways, in a way that is a part of the sound human understanding [gesunder Menschenverstand] and in a philosophically more refined way.

The first justification can be stated as follows. To the essence of the possible as such–to the possible in abstracto–belongs not only to be thinkable, but also to be thought. The possible is not a mere ‘nothing’. It is ‘something’ that can exist, but it is not exerting its aptitude to exist. In virtue of such a capacity to be, everything possible is necessarily thinkable. In his review of Kant’s The only possible argument, Mendelssohn had indeed written (1764b, p. 604), “In mere possibility the thing itself does not exist, but mere relations of something to something are posited according to the principle of contradiction”. But as a “possible concept” the merely possible can only exist as the object of an act of thinking, as “the alteration of a subject” or as “a thought in a thinking being”, according to Mendelssohn’s perhaps somewhat misleading formulae (1785, p. 142; 2011, p. 104). The merely possible only has, according to another Mendelssohnian expression, an ‘ideal existence’, an existence as idea, or rather, as ideatum, as the correlatum of an act of ideation. In short, the esse of the possible thus understood is percipi or, if preferred, cogitari. In reviewing Kant’s 1763 book, Mendelssohn had also written (1764b, p. 604), “The merely possible is to us human beings nothing more than an idea of our intellect”. Twenty-four years later, in his Morning hours, the philosopher states this same truth in a general way, “Each possibility must be thought as a possibility” (Mendelssohn, 1785, p. 142; 2011, p. 104).

As regards the actual, it should be noted that Mendelssohn, because of the starting point of his argumentation–the imperfection of our self-knowledge–does not consider the actual as such, in abstracto. He regards the actual as a predicate that has to truly belong to a thing, in this case, to my own make-up–a predicate, however, unknown to me. Thus, Mendelssohn refers to an actuality that is supposed to be true. From this standpoint, the philosopher’s claim is evident, “also each actuality, if it is supposed to be true, must be known and conceived to be true by some sort of being” (1785, p. 142; 2011, p. 104). The very essence of truth demands that everything actual that is supposed to be a true predicate of a thing must be thought of as actual. Mendelssohn confirms,

A concept must correspond to the thing; each object must be depicted in some sort of subject, each paradigm imitated in some sort of mirror. A thing without a concept has no truth; truth without some sort of entity assured of it does not bring the slightest bit of evidence with it and is thus no truth (1785, p. 142; 2011, p. 104).

However, Mendelssohn does not seem to consider this last explanation definitive. He acknowledges that “What could still be unclear to some extent in this chain of inferences is the proposition that everything actual must be thought by a thinking being” (1785, p. 143; 2011, p. 104). Thus, he proposes a new, more elaborate justification of the proposition in question. It can be stated and reconstructed where needed as follows.

On the one hand, it must be recognised that everything actual is also eo ipso thinkable. On the other hand, the merely possible can be also considered in concreto, i.e. as something outside the mind, as “a capability, facility, predisposition for something” that an actually existing being has (Mendelssohn, 1785, p. 143; 2011, p. 104). As a potentiality or disposition of a being, the merely possible has a necessary ground in the present make-up of an actually existing being. The actual constitution of a thing can be conceived to be otherwise, thus allowing a number of capabilities to be attributed to it.

If we attribute a possibility to any object at all as part of its make-up, then we are merely saying that, on the basis of the present make-up of the object, it can also be conceived how, in other circumstances, it would take on that property that was ascribed to it as possible (Mendelssohn, 1785, p. 144; 2011, p. 105).

If this were not so, it would obviously be absurd to ascribe to an actually existing thing as a feature of its objective make-up what does not actually exist.

This truth, however, reveals again not only that “A possibility that is not thought is a veritably impossible thing (ein wahres Unding)”, but also that “everything actual must not only be thinkable but also thought by some being or other” (Mendelssohn, 1785, p. 145; 2011, p. 106). Certainly, thinking of a possible different make-up of an actual thing necessarily requires thinking of the present actually determined make-up of the thing. Therefore, Mendelssohn concludes, “To every real existence, there corresponds an ideal existence in some subject or other; to each thing, a representation” (Mendelssohn, 1785, p. 145; 2011, p. 106).

Mendelssohn puts this argument in another way: “[I]f a[n actual] thing something thinkable is not supposed to be actually thought by any thinking being” (1785, p. 145; 2011, p. 106), then a contradiction arises. To affirm that the thinkable is never thought of by any intellect is tantamount to affirming that the thinkable can never be thought, i.e. that the thinkable is not thinkable. Thus, “Without being known, nothing is knowable; without being noticed, no characteristic mark, without a concept no object is actually on hand” (Mendelssohn, 1785, p. 145; 2011, p. 106).

If these reasons are cogent, then Mendelssohn’s claim (step 4) is entirely justified: “Th[e] agreement between a thing and [its] concept knows no exception” (1785, p. 145; 2011, p. 106). Everything actual and everything possible in the thing has to correspond to a concept in some thinking being. As Mendelssohn writes, “Each characteristic mark, each distinguishing sign of the thing, just as it is to be found in the latter, must be thought in all its truth by some thinking being, with the highest possible distinctness, completeness, and exhaustive detail” (1785, p. 145; 2011, p. 106).

Having proved (as a consequence of steps 4 and 5) the necessity of the existence of a thinking being (step 6), Mendelssohn completes his chain of inferences with a last stretch of argumentation, its ‘essential moment’ (steps 7 and 8), to determine the nature of such a thinking being. It is evident that the sum total of the possibilities and actualities belonging to my make-up cannot be thought in a supremely perfect way, i.e. in such a way that the ‘concept’ harmonises with the ‘thing’, by any limited being, not even by the sum of all of them. It is therefore necessary to admit that, in Mendelssohn’s own words (1785, p. 147; 2011, p. 107), “Complete and exhaustive concepts can only be found in a perfect intellect, and a perfect intellect does not exist without a perfect will, nor supreme discernment without the freest choice and most effective expression of power”. In short, there has to be a perfect intellect, i.e. God exists.

How did Kant and his disciple Jakob judge this novel proof of the existence of God?

3. Kant’s general assessment of Mendelssohn’s new theistic proof

In addition to the judgement on Mendelssohn’s last work set out in “Some remarks” on Jakob’s book on Morning hours (Kant, 2007), Kant pronounces explicitly, albeit very briefly, on the Mendelssohnian new proof on two occasions. The first one, specifically and privately, in the letter to Christian Gottfried Schütz at the end of November 1785.[5] The second one, more generally and publicly, in a footnote of his 1886 essay What does it mean to orient oneself in thinking?

Admittedly, from a Kantian standpoint, Mendelssohn’s new proof does not seem to fit neatly into any of the “only three kinds of proof for the existence of God possible from speculative reason” (KrV, A590-591/B618-619; Kant, 1998, p. 563) discussed by Kant in the Critique of pure reason. The proof does not “abstract from all experience” nor is it “empirically grounded on an experience that is only indeterminate” (KrV, A590-591/B618-619; Kant, 1998, p. 563). It certainly begins “from a determinate experience” (KrV, A590-591/B618-619; Kant, 1998, p. 563). But such an experience is not the one that Kant seems to be thinking of, namely, the experience of the “manifoldness, order, purposiveness, and beauty” present in the world (KrV, A622/B650; 1988, p. 579). It is rather the experience of the incompleteness, inadequacy, deficiency, and disharmony of our self-knowledge.

However, if instead of attending to its starting point, its nervus probandi is considered as interpreted by Kant, the proof seems to fit best into the classification of all possible arguments for God’s existence proposed in 1763 book The only possible argument in support of a demonstration of the existence of God. As is well known, in this work Kant claims that “[a]ll arguments for the existence of God must derive from one or other of two sources: either from the concepts of the understanding of the merely possible, or from the empirical concept of the existent” (BDG, AA 2:155; 1992, p. 195). Mendelssohn’s proof certainly starts from experience, but, according to Kant, its real source, the genuine nerve of the proof lies in the concept of the merely possible. On this view, the indubitable existence of imperfect knowledge of ourselves only gives occasion for inferring from the concept of the merely possible the existence of God. Specifically, Mendelssohn’s argument proceeds not “from the possible as a ground to the existence of God as a consequence”, but rather “from the possible as a consequence to the divine existence as a ground” (BDG, AA 2:156; Kant, 1992, p. 195). According to the pre-critical Kant, this is, in fact, the only path by which “the proof of God’s existence ought to be sought” (BDG, AA 2:163; 1992, p. 201) since any other path proves impracticable. Consequently, it is reasonable to think that Kant interprets Mendelssohn’s proof as a new and original version of what in the 1763 book is regarded as the “only one argument which enables us to apprehend” the existence of the “only one God” (BDG, AA 2:162; 1992, p. 201).

Kant indirectly confirms the place that Mendelssohn’s proof should occupy. In the letter to Schütz he specifically describes the core of Mendelssohn’s argument as the transition from “the conclusion that something is conceivable only if it is actually conceived by some being or other, and that without a conception no object really exists”, to the assertion that “an infinite and at the same time active understanding must really exist, since only in relation to it can possibility or reality be meaningful predicates of things” (Br, AA 10:428; Kant, 1999, p. 238). Furthermore, in his 1786 essay on orientation in thinking he considers that “all the proofs of the worthy Mendelssohn in his Morning hours” (thus also the new one) are ultimately based on the view that “reason needs to presuppose reality as given for the possibility of all things” (WDO, AA 8:138n; Kant, 1996, p. 11n.).

To understand Kant’s assessment of Mendelssohn’s proof, it is therefore necessary to recall the singular destiny of Kant’s own ‘only possible argument’. What this argument tries to investigate is, in Kant’s own words, “whether the fact that something is possible does not presuppose something existent, and whether that existence, without which not even internal possibility can occur, does not contain such properties as we combine together in the concept of God” (BDG, AA 2:157; 1992, p. 196). In 1763 Kant thought he could furnish this argument for God’s existence from possibility (BDG, AA 2:70-92; 1992, pp. 116-136). Certainly, something is ‘formally’ or ‘logically’ possible if its concept is thinkable by not containing a contradiction. But nothing would be ‘materially’ o ‘really’ possible if, additionally, there were nothing actual, no material to be thought (BDG, AA 2:78; 1992, p. 123). The last substratum of all possibility ultimately leads to the admission of an ens realissimum, i.e. of God, whose objective existence is supposed to be demonstrable.

The subsequent critical turn of his philosophy convinced Kant of the impossibility of such a proof. Certainly, Kant always maintained the necessity of admitting a ground for the inner possibility of things and the consequent necessity of accepting an ens realissimum. However, this necessity is no longer conceived as an objective necessity of things, but only as a subjective necessity of thought. The ens realissimum “is a transcendental ideal which is the ground of the thoroughgoing determination that is necessarily encountered in everything existing” (KrV A576/B604; Kant, 1998, p. 556). In the Critique of pure reason, God ceases to be a constitutive principle of reality and becomes a merely regulative principle of thought. This change of view of the 1763 proof perhaps helps to explain why ‘the only argument’ is not mentioned again as such in the Critique of pure reason and why the name it receives in the pre-critical work, that of ‘ontological argument’, is used exclusively in the first Critique to designate a different argument, the Anselmian argument or ‘Cartesian argument’.[6] Kant’s change of mind also makes it understandable why even the privilege of being “the only possible argument” [der einzig mögliche Beweisgrund], “which no human reason can bypass”, is attributed in the later work solely to the Cartesian ontological argument (KrV, A623/B653; Kant, 1988, p. 580).

Not surprisingly, Kant judges Mendelssohn’s proof in the light of his later assessment of his own “only possible ground of proof for the existence of God” (KrV, A623/B653; 1988, p. 580). For this reason, Kant can spare himself any discussion of the particulars of the Mendelssohnian argumentation and content himself with pointing out concisely both the fundamental illusion undermining it and the utility that, despite its invalidity, the argument still has. It is also understandable that Kant could leave to disciples, especially to Jakob, the task of refuting more thoroughly Mendelssohn’s attempt to renovate rational theology after the criticisms of transcendental idealism.

As expected, in Kant’s view, Mendelssohn’s new proof achieves nothing by way of demonstration. With it his author only presents “the subjective conditions of the use of our reason” (Br, AA 10:428; Kant, 1999, pp. 237-238), namely, “a subjective ground of necessity, i.e. a need in our reason itself to take the existence of a most real (highest) being as the ground of all possibility” (WDO, AA 8:138n.; Kant, 1996, p. 11n.). The “illusion” in which the proof incurs is then the failure to distinguish “the merely subjective conditions” of the employment of reason “from those from which something valid about objects can be inferred” (Br, AA 10:428; Kant, 1999, pp. 238). According to Kant, Mendelssohn’s new proof ultimately makes the same mistake as the Cartesian proof. In both, “subjective grounds for presupposing something for the use of reason (which always remains a ground only within an experiential use) is taken to be objective – hence need is taken for insight” (WDO, AA 8:138n.; Kant, 1996, p. 11n.).

The utility that Kant nevertheless attributes to this new proof, as to all the others put forward by Mendelssohn, is twofold. Firstly, it “provides us with the most splendid occasion and at the same time challenge to subject our faculty of pure reason to a total critique” (Br, AA 10:428; Kant, 1999, pp. 238). Secondly, it shows the great importance of “a holding of something true on subjective grounds of the use of reason – if we lack objective ones and are nevertheless necessitated to judge” (WDO, AA 8:138n.; 1996, p. 11n.). Although, as Kant observes, “we must not give out what is in fact only a necessary presupposition as if it were a free insight” (WDO, AA 8:138n.; 1996, p. 11n.).

4. Jakob’s particular criticisms of Mendelssohn’s new theistic proof

In contrast to Kant’s sparse indications, Jakob devoted the entire Eleventh Lesson of his Examination of Mendelssohn’s Morning hours or of all speculative proofs for the existence of God in lectures to the “Examination of the new Mendelssohnian proof”. Not in vain this work is intended “to contrast honestly and impartially the reasons of the old school for the existence of God, which Mendelssohn certainly put forward in all their strength, with the Kantian assessment of them” (Jakob, 1786, pp. i-ii). Specifically, the book tries to verify “whether, through Mr Mendelssohn’s sagacity, the old cause has been gained, and whether, through the various new ways and turns he used, the limits of pure reason have really obtained more extension than Mr Kant has pointed out to them” (Jakob, 1786, pp. i-ii).

Jakob puts forward Mendelssohn’s new proof in three ways: succinctly “in the order chosen by himself”, elaborating on the claim that a being must be thought because it can be thought, and even putting the argument “in scholastically correct inferences” [in schulgerechte Schlüsse] (Jakob, 1786, pp. 230-237, 240-243). In all these presentations the ‘real’ [eigentliche. nervus probandi of the argument is identified as the claim that “everything actual must be thought, precisely because everything thinkable must necessarily be thought” (Jakob, 1786, p. 240). One of the characters in Jakob’s book says a few words in this regard that combine radical opposition to the proof with intellectual respect for its proponent, “I would certainly say that the transition is sophistical if the proof did not come from Mendelssohn” (Jakob, 1786, p. 240).

In order to explain the objections justifying this judgement, it is convenient to quote all the ‘scholastically correct inferences’ with which Jakob summarises Mendelssohn’s argumentation. The proof itself is stated by means of the following syllogism:

Everything possible must be thought as possible and everything actual as actual.

Now, only the most perfect intellect can think everything possible and everything actual.

Consequently, there is a most perfect intellect (Jakob, 1786, pp. 240-241).

Jakob provides Mendelssohn’s proof of the claim that ‘the possible must be thought’ through the following two inferences. The first reads:

Possible is everything that can once objectively become actual under certain circumstances.

Now, that which can become actual is not yet actual.

Consequently, the possible is not actual, or it is not to be found as an objective property of things (Jakob, 1786, p. 241).

The second is as follows:

What is not objectively actual has only an ideal existence.

Now, possibilities are not objective actualities.

Consequently, they have a merely ideal existence, or they are merely in the mind, and otherwise nothing.

All possibilities must therefore be thought, or they are otherwise nothing (Jakob, 1786, pp. 241-242).

Jakob also recaps Mendelssohn’s proof of the claim that ‘the actual must also be thought’ in two inferences. The first reads,

Everything actual must be at the same time thinkable, or (which is the same) possible.

Now, everything possible must also actually be thought [by virtue of the second inference concerning the possible].

Consequently, also everything actual (Jakob, 1786, p. 242).

The second inference about the actual is as follows:

If the actual were not thinkable or not possible, it would have contradictory predicates.

What has contradictory predicates is nothing.

Consequently, the actual would be nothing, which is absurd. The opposite, therefore, must be true, and the actual must be thought as well as the thinkable (Jakob, 1786, pp. 242-243).

In examining these lines of reasoning, Jakob discovers several flaws that render them invalid. Mendelssohn’s argumentation consequently incurs either a petitio principii or, maybe more properly, a quaternio terminorum.

One of the characters intervening in Jakob’s book proposes a very obvious and immediate objection against the claim that everything actual, precisely because it is thinkable, must also be actually thought by an intellect. The objector says indeed, “That everything actual must be thinkable seems obvious to me, but it can only actually be thought on the condition that there is a thinking being. But the existence of the latter must first be proven” (Jakob, 1786, p. 237).

The error underlying this begging the question is, according to the aforementioned character of Jakob’s book, the confusion between the possible and the thinkable. The possible “merely states that all actual objects are subject to the logical law, that their objective predicates must not cancel each other out” (Jakob, 1786, p. 238). It is the objective possibility corresponding to things. It requires that things be subject to the law of contradiction understood as an ontological principle. The thinkable, instead, “here merely expresses a relation of the cognising being itself and demands that all beings to be cognised do not contradict this characteristic” (Jakob, 1786, p. 238). It is the subjective possibility, i.e. a condition only under which the human understanding can cognise the actual. It requires that our thoughts be subject to the law of contradiction understood as a logical principle. Thinkability is, therefore, an extrinsic denomination [denominatio extrinsica] to things. The confusion of objective possibility with subjective possibility would therefore explain Mendelssohn’s begging the question, since “many things can be objectively actual whose objective possibility and actuality are not cognised by any thinking being” (Jakob, 1786, p. 238). “The clock can always show its hours without anyone knowing it” (Jakob, 1786, p. 239).

However, to better highlight the error of Mendelssohn’s claim that “everything actual must be thought, precisely because everything thinkable must necessarily be thought”, the character in the book who expresses Jakob’s own Kantian position proposes a new clarification of the “objectively possible”, comparing it with the “actual” (1786, p. 244). The proposed explanation faithfully reflects indeed Kant’s teaching on the postulates of empirical thinking in general.

Thus, Jakob defines “possible”, in an objective sense, as “everything that can ever be cognised by us under certain conditions either directly or by rules of experience in space and time” (1786, p. 244). An object of possible cognition, apart from agreeing with the laws of thought, i.e. apart from being subjective or logically possible, must be also objective or really possible, i.e. it must agree, in Kant’s terms, “with the formal conditions of experience (according to intuition and concepts)” (KrV, A218/B265; cf. BXXVIn.; 1998, p. 321, cf. 115n). “Actual” is, instead, according to Jakob’s description, “everything that corresponds to our sensation, it is either directly perceived itself or is related to it according to the laws of experience” (1786, p. 244). Actual is indeed, in Kant’s own words, “[t]hat which is connected with the material conditions of experience (of sensation)” (KrV, A218/B266; 1998, p. 321).

In the light of these clarifications, if by “possible” the merely subjective or logical possible is meant, “then it is evident that everything possible must be thought, because it consists of mere thoughts, and these cannot float around outside thinking beings” (Jakob, 1786, p. 246). But if by “possible” the objectively or really possible is meant, i.e. “those laws according to which things are put together and attain their actuality, or even only the objects which can ever come to actuality, then there is no telling why these would have to be thought” (Jakob, 1786, p. 246).

Mendelssohn’s proof of the claim that ‘everything actual must be thought’ might then be stated as follows:

Everything actual must be at the same time (really or objectively) possible.

Now, everything (logically or subjectively) possible must also actually be thought.

Consequently, also everything actual must also actually be thought.

The proof is, as is easily seen, a patent quaternio terminorum. “For the actual”, Jakob explains, “must be objectively possible, but not subjectively possible, i.e. it must be in accordance with the laws, but it is not necessary for a thinking being to represent its possibility or actuality” (1786, p. 247).

Thus, like Kant, Jakob does not grant Mendelssohn’s new proof any demonstrative value, although he does recognise a certain ‘latent truth’ in it. Indeed, considered from the results of the Critique of pure reason, Mendelssohn’s claim that everything actual must be thought by a thinking being is fully correct if it refers to the phenomena. In this case, the statement means that “the whole world of the senses exists only subjectively as a representation in us, and all laws and rules are not objective, but our intellect gives them, and through them makes possible at first the objects of its knowledge” (Jakob, 1786, p. 250). But whether the things in themselves “must be thought by some infinite intellect (as is assumed here without reason) we cannot say at all, since we are neither acquainted with the nature of objects nor with the nature of an infinite intellect” (Jakob, 1786, pp. 251-252). Therefore, Jakob concludes, “this new Mendelssohnian attempt to prove the existence of a supreme being from speculative reason is also fruitless and futile” (1786, p. 262).

5. Conclusion: A case of misunderstanding

Interestingly, neither Kant nor Jakob seem to attach any importance to the starting point of Mendelssohn’s new proof–the imperfection of our self-knowledge. Since its nervus probandi asserts that the possible and the actual must be actually thought, Kant interprets the proof as a new and vain attempt to conclude that the actual is the objective ground for the possible. Jakob, for his part, seeks to discover in it the confusion of two quite different kinds of possibilities. The exposition of the proof has clearly highlighted, however, that Mendelssohn does not try to properly establish an objective ground for the actual nor does he merely identify the possible with the thinkable, since he also understands the possible as the real potentiality of a being.

Kant’s and Jakob’s understanding of Mendelssohn’s new theistic proof leads to two main and interrelated conclusions concerning the value of their criticisms. The first one is that both Kant and Jakob seem to examine the Mendelssohnian proof by taking the results of the Critique of pure reason for granted. Kant, indeed, considers it in the light of his doctrine of the transcendental ideal of reason and Jakob studies it on the basis of the theory of the postulates of empirical thinking and the distinction between phenomena and noumena. Thus, far from the truth of these doctrines being indirectly confirmed by the refutation of the proof, it is the truth of these doctrines on which the refutation of the proof rests. But if this is so, the suspicion is inevitable that neither Kant nor Jakob have revealed the internal flaws of the proof but only its radical incompatibility with the basic tenets of transcendental idealism.

The second conclusion is that neither Kant nor Jakob seem to have grasped the true nature of Mendelssohn’s proof and its originality. The starting point of the proof reveals its nature: it is a proof from undoubtedly known truths. Its originality lies in the fact that the undoubtedly known truths from which it starts are not the highest truths of which we are capable, but truths that reveal the imperfection of our knowledge. Thus, the proof does not start from the so-called ‘eternal truths’, as is the case with the arguments of Augustine, Leibniz or Lambert, but from the truths that we undoubtedly know that we do not know about ourselves. It is a case, we might say, of docta ignorantia. I undoubtedly know, indeed, that there are many potentialities and actualities in my own being although I do not know what they are. If there were only those potentialities and actualities that I know as actually belonging to my being, I could not even exist, since such potentialities and actualities are not enough to determine my essence.

Consequently, Mendelssohn’s main concern can be stated as follows. How can a predicate be truly attributed to a being, if such a predicate is not an objective property of the being in question, either because the property does not yet exist, though it can exist (as in the case of the potentialities which I do not know about myself), or because the property is unknown (as in the case of the actualities which I do not know about myself)? Mendelssohn’s answer is, as we have seen, that such a property, since we know that it is truly attributed to us, but we cannot identify it either as possible or as actual, must necessarily be thought of as possible or as actual by some intellect. Since we know with certainty that properties that do not yet exist or that we cannot even know, and that, therefore, are not a ‘pure nothing’, must be truly attributed to us, such properties must be somehow present in some intellect. Such an intellect can ultimately be none other than the divine one. Therefore, the incompleteness of our knowledge reveals the existence of God.[7]

It is certainly disputable whether Mendelssohn is right in arguing that it is not enough for the possible to be thinkable, but that it must also be thought. It may well also be questioned whether Mendelssohn manages to justify without a shadow of a doubt his claim that the actual must be thought of as actual. Is this last statement true only as a consequence of the fact that the actual is also thinkable and that everything thinkable is also actually thought, according to the reason criticised by Jakob? Or is it true perhaps because the possible, existing only in thought, can only be thought of as a modification of the actual and therefore requires that the actual be thought of as actual, as Mendelssohn’s statements can perhaps be interpreted to mean?

Whatever the solution to these or any other doubts that may arise,[8] neither Kant nor Jakob seem to have responded to the intellectual challenge, if we may call it that, posed by Mendelssohn. The solid construction built by the critique of the pure use of reason prevented both thinkers from understanding the question that Mendelssohn ultimately seems to raise with his proof. The question can be formulated like this: how is truth possible regarding that which is not present to our consciousness, as is the case of the possibilities and actualities that are truly part of our being and that, because of the limitation of our intellect, are not thought at all by us?

Questions analogous to this continue to move reflection today. Certainly, Mendelssohn tries to explain, among other matters, that there are genuine truths concerning the possible, i.e. concerning that which does not yet exist at all. Thus understood, Mendelssohn’s question can be seen as a precedent for the question that Hans Jonas and Robert Spaemann have recently posed through independent paths: How is truth concerning facts about the past possible, i.e. concerning that which in no way is any longer actual? Both thinkers, maybe unaware of Mendelssohn’s previous attempt, have proposed in response a new proof (or a postulate) for the existence of God. Jonas (1992, pp. 173-189) calls it “a later supplement to the so-called proofs of God”. Spaemann (2007) denominates it “the last proof of the existence of God”. Whatever value may be attached to these arguments, these attempts seem to support a deep conviction of Mendelssohn. Far from thinking, as Kant argues, that there is one “only possible argument in support of a demonstration of the existence of God” (KrV, A623/B653; 1988, p. 580) Mendelssohn was convinced very early on that “no one who reveres God must take exception to the slightest basis of proof which brings only a modicum of convincing power with it” (1764a, p. 312; 1997, p. 292).


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1 Faculty of Philosophy. Complutense University of Madrid. Contact: ORCID: Work on this paper was carried out within the research project PID2020-115142GA-I00, financed by the Spanish Ministry of Science and Innovation.
2 See, for instance, Reinhold (1790, p. 142). For an overall exposition of Mendelssohn’s proofs for the existence of God (with mentions of Kant’s and some of his disciples’ assessments of the Morning hours), see Altmann (1975). Guyer (2020, pp. 75-102, 141-145) offers an interesting comparison between Mendelssohn’s and Kant’s positions facing the proofs for the existence of God. For an exposition of Mendelssohn’s proofs for God’s existence in the Morning hours and the Kantian project of “transformation of the proofs for the existence of God”, see König (2020, esp. pp. 90-129).
3 Altmann (1973, pp. 322-327; 1975, pp. 23-25) was the first to draw attention to this essay.
4 Kant’s ultimate identification of the criterion of determinate and indeterminate experience with the distinction between matter and form of experience makes it impossible to accept that in the Morning hours Mendelssohn agrees with Kant in classifying the possible kinds of proofs for the existence of God, as König (2020, pp. 91-92) argues.
5 However, Schütz published the content of this letter, without expressly naming Kant as its author, in his early review of Morning hours (1786, p. 56).
6 It is important not to confuse, therefore, the argument that the pre-critical Kant calls ‘ontological’ (i.e. the argument that proceeds “from the possible as a consequence to the divine existence as a ground”) with the Anselmian or Cartesian proof (i.e. the argument that proceeds “from the possible as a ground to the existence of God as a consequence”) (BDG, AA 2:156; Kant, 1992, p. 195), which in the Critique of pure reason is called “ontological” and described as the “proof of the existence of a highest being from concepts” (KrV, A602/B630). To avoid misunderstandings, the “only possible argument” is nowadays often called by Kant scholars “ontotheological proof” or “possibility proof” (KrV, A623/B653; Kant, 1988, p. 580).
7 In a recent essay, Atlas (2019) argues that Morning hours can be viewed as the final stage in a philosophy of imperfection that Mendelssohn had been developing throughout his life. In Mendelssohn’s last work, indeed, there are two arguments about imperfection: one–specifically studied by Atlas–tries to show that our imperfections are proof that we are not mere modifications of the divine being; the other–here considered–is that the imperfection of our self-knowledge is evidence for the existence of God.
8 It is worth noting the objection, inspired by Kant’s criticism of physico-theological proof, that König proposes against Mendelssohn’s new proof. According to König (2020, p. 108), Mendelssohn would have not proved in any way that “every finite being should have an infinite number of characteristics”. Consequently, “the intellect who comprehends all things in his thinking” “needs not be infinite at all, but only large beyond all measure” (König, 2020, p. 108). Mendelssohn would not then have proved the existence of a most perfect being, i.e. God. It is true that Mendelssohn (1785, p. 143; 2011, p. 104) claims that in the actuality of a contingent being “lie infinitely many characteristics”. However, as we have seen, his proof does not seem to rely so much on the need to know the quantity (no matter whether finite or infinite) of characteristics of a being, as on the need to know “[e]ach characteristic mark [...] with the highest possible distinctness, completeness, and exhaustive detail” (Mendelssohn, 1785, p. 145; 2011, p. 106), what is only available to an infinitely perfect intellect.
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