2. The Transcendental Ideal and the Possibility Proof

Kant’s pre-Critical possibility proof for the existence of God is first presented in 1755 (NE, 1: 395) and then elaborated in 1763 (OPA, 2: 77–92). The gist of the argument is as follows:

1. (1) The possibility of any individual thing presupposes not only a logical condition, i.e. the non-contradiction of its predicates, but also a material condition: the content of its predicates is given through some actual thing, either directly from predicates of an actual thing or indirectly derived from fundamental predicates of an actual thing (OPA, 2: 77–8).

2. (2) Using the notion of a material condition, Kant contends that the possibility of any individual thing is grounded in something actual (following Abaci, principle AP).

3. (3) If there are no actual things, i.e. nothing exists, no predicates can be given, and therefore nothing is possible.

4. (4) It is impossible that nothing is possible and therefore

5. (5) It is necessary that something exists.

6. (6) There exists one being grounding all possibilities. Since without this being nothing would be possible, this being exists necessarily.Footnote ⁴

7. (7) This necessarily existing ground of all possibility is God.

The argument is problematic in various ways, as pointed out by many commentators.Footnote ⁵ However, I will forgo here the examination of the various problems because my concern is Kant’s own rejection of the proof in the Critique of Pure Reason and the dialectical illusion he associates with it.

I will only make one point about (2) which is relevant for the rest of my argument. There are various interpretations about the nature of the grounding relation between the actual thing and the possible predicates. The debate revolves mainly around the question whether possible predicates are grounded in virtue of being instantiated in something actual or by some other means.Footnote ⁶ Abaci, for example, maintains that a predicate is made possible in virtue of its ‘instantiation’ in an existing thing (Abaci Reference Abaci2017: 265). It is true that a predicate’s being instantiated logically entails that it is possible. Under some accounts, instantiation might also be a condition for the epistemic fact that a predicate is knowable. Indeed, Kant sometimes expresses the grounding relation in epistemic terms, as the requirement for a material element to be given for thought.Footnote ⁷ But since the argument aims to derive an ontological claim from the grounding of possibility, the grounding relation is not only a logical or epistemological relation but rather a metaphysical one that entails some ontological assumptions about its relata.Footnote ⁸ Thus, apart from the question of ‘instantiation’, we need to consider other features of the relata to reveal what is entailed ontologically from the grounding of possibility. The grounded things are predicates of possible things and so are abstract entities, and therefore it is plausible to think also of the ground as an abstract entity. One way to characterize such a metaphysical grounding relation is as a sort of Platonism about the grounds of possibilityFootnote ⁹. I will argue that Kant’s realization that proving the existence of God from the grounds of possibility involves the reifications of concepts into abstract entities explains the ultimate rejection of the possibility proof, as well as the regulative significance of the corresponding conception of God.

Kant continued to value this proof in the Critical period.Footnote ¹⁰ In his lectures on rational theology given after the publication of the Critique, he continues to mention it favourably:

Although the proof ‘affords the most satisfaction’, Kant states that it cannot prove the objective existence of God, but only the subjective necessity of assuming it. But what prevents the proof from being apodictically certain? Kant’s transcendental idealism entails that knowledge is limited to what can be given in sensible intuition and the a priori conditions of experience. Hence knowledge about a supersensible object such as God is impossible. The natural question that arises is whether, beyond the restrictions of transcendental idealism on knowledge of the conclusion, there is a fallacy within the proof itself. Without locating such a fallacy, the proof could perhaps provide a counter-argument overriding the justifications for transcendental idealism, proving that we can know the existence of at least one object of traditional metaphysics independently from the conditions of possible experience.Footnote ¹¹

In the Critique itself, Kant does not explicitly mention the possibility proof among the arguments for the existence of God. But a similar line of thought underlies his discussion of the theoretical conception of God as the ens realissimum (the most real being) in section two of ‘The Ideal of Pure Reason’, ‘the transcendental ideal’ (A571–84/B579–662). The construal of this conception begins here not from the grounding of possibility, but from the principle of thoroughgoing determination:

To put it simply, this principle prescribes what it is like to know everything there is to know about something, so that this knowledge would suffice ‘to cognize a thing completely’, i.e. as an individual object, solely through its concept (A573/B601). The principle of contradiction places logical constraints on concepts so that at most only one of a pair of contradicting predicates can apply to each concept of a thing. The principle of thoroughgoing determination, on the other hand, specifies the conditions for the individuation of a thing through its concept, i.e. in terms of a set of predicates: that one of each pair of all possible contradicting predicates applies to the object. According to the principle of the identity of indiscernibles, objects with the same set of predicates are identical, and hence the complete set of predicates uniquely identifies an individual object. Thus determining the complete set of predicates presupposes a totality of all possible predicates, ‘the entire storehouse of material from which all possible predicates of things can be taken’ (A575/B603). The sum total of all predicates is also the whole of reality (omnitudo realitatis) when considering positive predicates as signifying a reality and negative predicates as lack of reality. Each concept of an individual thing can then be determined by selecting a subset from this whole. Till now, the result is entirely conceptual, without making an ontological claim:

What enables the ontological shift in the final step is regarding the ground of the whole of reality as a single object. The ground of all reality should be thought of as prior to its parts and not as a mere aggregate of parts. Hence there is a shift from the notion of a sum total that can be limited to something that grounds all realities:

One reason for this shift is reason’s demand for an unconditioned condition. An aggregate of independent parts would be conditioned by their parts and therefore not unconditioned. A second reason appears at the end of the last quote: the ground of all reality is also the ground of sensible realities, and those can only be grounded in the highest being but not included in it without sliding into Spinozism. What is important to note is that this shift brings us closer to the ontological commitment of OPA: the transcendental ideal is not only a concept of an aggregate of predicates but an ontological ground for predicates, which means that they are being regarded ontologically as abstract entities.

When taken as a single object, the ground of all reality is the most real being, the ens realissimum (A576/B604), or in other words, God (A580/B608). Since the conceptual individuation of things explains their possibility qua individuals, God as the condition for thoroughgoing determination is also ‘the material ground of all possibility’ (A573/B601; cf. A576/B604). The latter is the same term used in the possibility proof. The same ‘all of reality’ required for thoroughgoing determination of things is also the material ground, the real element of their possibility in the pre-Critical sense. Thus the affinity between this argument and the construal of the ideal of reason in the Critique is evident, textually and conceptually.Footnote ¹²

3. The Error of Subreption in the Possibility Proof

Since the construal of the ens realissimum is similar to the reasoning of the possibility proof, it is plausible to look for criticism of the proof in the analysis following this construal. Kant makes it clear that forming the concept of the ens realissimum does not include an assertion about the existence of its object:

The conclusion of the possibility proof, however, does assert the actual existence of God, and from similar considerations to the ones leading to the construal of the ens realissimum. Where does this erroneous conclusion come from? After presenting how reason construes the ens realissimum, Kant sets the task to identify the source of the error of inferring its existence, the dialectical illusion:

Kant’s account of the nature of the error that leads reason to regard ‘all the possibility of things’ as grounded in one being is packed into the last two paragraphs of section two and is very succinct and opaque. I will, therefore, analyse it in detail. The account begins with the starting premise of the possibility proof, the grounding of possibility in actuality (AP). Kant states that this principle is valid for the possibility of cognizing objects of experience:

Kant begins by alluding to the Postulates of Empirical Thinking (A218–26/B266–74).Footnote ¹⁴ In their legitimate empirical use, the categories of modality do not determine anything in the object itself but only the relation of thought to the object. Two elements can be discerned in this relation. First, the object has to correspond with the formal conditions of experience, the a priori forms of sensible intuition and the categories. Secondly, the content (or matter) of the sensible realities which belong to the object ‘has to be given’ (A581/B609), i.e. regarded as actual.Footnote ¹⁵ Thus the possibility of cognizing empirical objects presupposes the whole of experience as an actual sum-total of all sensible realities:

This sum total is then conceived as the condition of the thoroughgoing determination of empirical objects: ‘all possibility of empirical objects, their difference from one another and their thoroughgoing determination, can rest only on the limitation of this sum total’ (A582/B610). This line of thought is analogous to the starting point of the possibility proof. Also there, Kant distinguishes between the formal aspect of possibility (the principle of non-contradiction), and the material aspect, the actual ground of possibility (AP). But Kant contends that this analogy is deceptive: while AP is true for cognizing the possibility of empirical objects, it is not applicable to grounding the possibility of things in general:

This is a case of ‘transcendental subreption’, by which a principle of possible experience is illegitimately used to gain knowledge of things independently of possible experience. i.e. things-in-themselves.Footnote ¹⁶

As Abaci shows, in its legitimate use for objects of experience, AP is an epistemological principle: for cognizing the possibility of empirical objects as fully determined regarding all realities, we presuppose the actuality of all the empirical realities in one sum total.Footnote ¹⁷ The metaphysical error lies in substituting an epistemological AP applicable to the cognition of objects of experience with an ontological AP applied to things in general (Abaci Reference Abaci2017: 285). But a complete critique of the possibility proof requires not only identifying the metaphysical error but also explaining the transcendental illusion that motivates reason to commit the error. Abaci focuses mainly on the error, and in continuity with Grier claims that the explanation of the illusion comes only in section three of the Ideal chapter. In the next section I lay out my alternative account of the illusion, and in section 5 I argue against Grier’s account.

4. Collective Unity and the Formation of the Transcendental Ideal

In what follows I will show that there is an implicit account of a motivating illusion already in section two, in the notion of collective unity which comes up in the last paragraph of the section:

The subreption that transforms the epistemological AP into an ontological AP is manifested by hypostatization, i.e. regarding the idea of a ground of possibility as a real object. I take the ‘because’ in the above sentence to indicate a purposive explanation: the subreption is motivated by an interest in transforming distributive unity into collective unity. What is the difference between these kinds of unity? In the Appendix to the Dialectic, Kant describes them as follows:

The unity imposed by the understanding on the sensible manifold is distributive, while collective unity is a unity of reason, imposed on concepts (cf. Guyer Reference Guyer and Guyer2003: 5). According to their seat correspondingly in the understanding or in reason, we can make the following distinction. The distributive unity of the understanding is generated by the relations among objects of experience, the way in which each object is constituted by its relations to all other objects and the cognizing subject.Footnote ¹⁸ This unity is therefore distributed equally among its parts. The indeterminate whole composed thereby is only thought as an open series enabling the search for further relations without a definite endpoint. However, as a faculty of inferences, reason demands a different kind of unity, a collective unity of a system derivable from a single ultimate principle:

Although the paragraph at the end of section two refers to the ‘collective unity of a whole of experience’ leading to an ‘individual thing containing in itself all empirical reality’, we should note that this collective unity is not a necessary condition for the possibility of experience but a mere idea of reason. Since collective unity originates in the faculty of reason, it requires something beyond what is available through the synthesizing function of the understanding regarding sensible intuition.Footnote ¹⁹

This ‘collective unity of a whole of experience’ is then ‘by means of the transcendental subreption … confused with the concept of a thing that stands at the summit of the possibility of all things, providing the real conditions for their thoroughgoing determination’ (A583/B611). What motivates this confusion? My claim is that the confusion, the error of subreption, is motivated by the interest of reason in collective unity. As noted above, the unity of experience is merely distributive, the unity resulting from the spatial, temporal and categorical relations between the objects of experience. But the interest of reason is to regard any manifold as a collective unity, i.e. a unity in virtue of being derived from a single principle. Since the unity between the actual objects of experience cannot be collective but only distributive, reason presupposes unity in what grounds the possibility of the objects of experience. The ontological ground of possibility is amenable to the unity of reason because it is an abstract entity that can be thought through the reification of concepts.Footnote ²⁰

Thus an important step in the course of reason concluding in the existence of the ens realissimum is the reification of a concept that unites all other concepts systematically as an actual object that grounds all possibility of things. This is suggested in a footnote: ‘This ideal of the supremely real being, even though it is a mere representation, is first realized (realisiert), i.e., made into an object (Objekt), then hypostatized’ (A582–3/B610–11, n.).

The hypostatization (assigning existence) of the ens realissimum is the erroneous conclusion, but there is a prior step mentioned in this sentence, that of ‘realization’, i.e. thinking it as an object,Footnote ²¹ or what I called above reification. Once it is conceded that the Ideal designates an object that grounds all possibility, it is natural to infer its necessary existence, as without it nothing would be possible.

Let us summarize Kant’s account of the errors that follow from the construal of the ens realissimum:

1. 1. A principle valid for the possibility of cognizing empirical objects is applied to the possibility of things in general for which it cannot be demonstrated to be valid (the dialectical subreption of AP).

2. 2. The unity of experience which is only distributive is transformed into a collective unity of a system of concepts, in accord with an interest of reason.

3. 3. Using this ontological AP, reason infers a single actual object which is the unconditioned ground of the possibility and collective unity of all things in general – God.

What is the relation between (1) the dialectical subreption of AP and (2) the error of substituting collective unity for distributive unity? My claim is that there is a rational interest regarding the unifying distributive relations between empirical objects as grounded by the collective unity of relations between concepts. Thus reason is tempted to take its interest in (2) as objective, justifying the ontological commitment entailed by the subreption (1). Regarding the collective unity of reason as objective means making concepts into objects: making something which signifies only the relation ‘of an idea to concepts’ into an ‘objective relation of an actual object to other things’ (A579/B607). This leads to a metaphysical error when the objects thereby produced, purely intelligible objects, are regarded as existing. In section 6 I will expand on the nature of these objects and show how they function in the construal of the ens realissimum and the possibility proof.

The following note provides additional evidence that it is this interest in collective unity which ultimately motivates the formation of the ens realissimum:

The ens realissimum is construed for the sake of an unconditioned ground of the unity among all possibilities, i.e. unity among the essences which provide the ‘matter’ for all possible things. Hence the metaphysical error arises from thinking that the unity of concepts prescribed by reason has to be grounded in actual intelligible objects: ‘where the universality of thinking through reason is taken for a thought of a totality of the possibilities of things’ (R5553, 18: 224). In section 7, I show how this kind of unity receives a regulative status.

5. Transcendental Illusion and Section Three of the Ideal

According to Grier, the construal of the ens realissimum in section two of the Ideal chapter does not include an explanation of the illusion motivating the error, which comes only in section three. Thus she claims ‘that section 2 and 3 constitute two stages in a single extended argument’.Footnote ²² The following passage at the beginning of section three supports this reading:

Kant claims that the need to assume the material ground for thoroughgoing determination presented in section two is not persuasive enough to form belief in the actual existence of God without another motivation. This other motivation for belief in the existence of God is the argument that begins with contingent existence known through experience and concludes that there is something that exists necessarily, a cause for the whole series of contingent things:

The existence of each contingent thing is caused by something else. The general interest of reason is to complete the regress of causal conditions, and therefore it postulates an unconditioned cause for the whole series. This type of inference corresponds to the generic form of a transcendental illusion that Grier identifies in all the arguments of the dialectic: a transition from the legitimate yet subjective principle of reason, ‘Find for the conditioned knowledge given through the understanding the unconditioned’ (P1), to the illusory principle, ‘If the conditioned is given, the whole series of conditions, subordinated to one another – a series which is therefore itself unconditioned – is likewise given’ (P2).Footnote ²³ P2 is here applied to the causal series of contingent things, resulting in a necessary first cause. Kant then continues to assert that only the unjustified combination of the two lines of thought, the first from the principle of thoroughgoing determination to the ens realissimum, and the second from the series of contingent things to their non-contingent cause, leads to the illusion of rational theology in its full force. Reason is compelled to assume the unconditioned cause for contingent things but has no insight into its nature. The concept of the most real being is then introduced as ‘best suited to the concept of an unconditionally necessary being’ (A587/B615). The ens realissimum is best suited for a concept of an unconditioned cause because it is unconditioned also in another sense, as the most real being which grounds the possibility of everything else.

According to Grier, it is only the need for an unconditioned causal ground of existence added in section three that accounts for the dialectical illusion motivating the hypostatization of the ens realissimum. Footnote ²⁴ Yet her account does not explain the relations between the series of errors detailed in section two, but only the appeal of using its final conclusion for a different purpose.Footnote ²⁵ Therefore, when considering the account of these errors as an implicit criticism of the possibility proof, locating the motivating illusion only in section three cannot explain how the pre-Critical Kant thought the possibility proof was convincing in the first place.

Moreover, the argument presented in section three for a necessarily existing cause of all contingent things is a version of a cosmological argument for the existence of God. Given that the construal of the ens realissimum in section two correlates with the pre-Critical possibility proof, locating the illusion only in section three would amount to the claim that the force of the possibility proof depends on an implicit appeal to the cosmological argument for supplying the concept of necessary existence. But could Kant be committed to this position? In OPA he explicitly maintained the opposite view: the possibility proof is the ‘only possible argument’ and superior to all other arguments, including the cosmological argument from contingency.

The specific criticism of the cosmological argument in OPA (2: 158) is similar to the one levelled in the Critique (A604–7/B632–5), and shows that it implicitly depends on the Cartesian ontological argument. What is relevant for the present purpose is Kant’s claim that the possibility proof is superior to the cosmological proof precisely with respect to the notion of necessary existence. The possibility proof renders the necessary existence of God intelligible because it proceeds from an analysis of the concept of absolute necessity as that whose non-existence cancels all possibility, while the cosmological proof appeals to the merely negative notion of the non-contingent:

I believe that it is unlikely that Kant changed his mind about the independence of the possibility proof from the cosmological proof. If this is the case, then there should be another kind of motivating illusion than the one Grier locates in section three, which explains why the mere conception of the transcendental ideal is mistaken for a proof from the ground of possibility. As I have shown in the previous section, this illusion stems from the interest in the collective unity of all possibilities. We should note that the collective unity of the grounds of possibility is also a form of an unconditioned ground, and hence stemming from the general illusory principle P2, but it is not the unconditioned ground of existence described in section three.

But how are we to understand Kant’s statement about the lack of convincing power of the procedure in section two which correlates with the possibility proof? I suggest we take it as a historical-anthropological observation rather than logical. The cosmological argument is more common in human history because it begins with ordinary experience and does not assume sophisticated metaphysical reflections:

Once human reason is led from the causal series of changes to the idea of a first cause, then further philosophical reflection can supply the idea of a most perfect being as a suitable candidate for that cause. But the logical course of reasoning as exemplified in OPA and still supported in the lectures as subjectively valid does not depend on the cosmological argument and is also not motivated by it, but by another demand of reason, that of collective unity. The next sections will elaborate on the ontological commitment to intelligibilia motivated by this interest and its relation to the regulative principle of systematic unity.

6. Intelligibilia and the Ontological Commitment of the Ideal

The error of subreption is that of taking the ‘empirical principle of our concepts of the possibility of things as appearances to be a transcendental principle of the possibility of things in general’ (A582/B660). What is meant by things in general? In several other places, the term ‘things in general’ is followed by ‘and in themselves’ (B298/A238, B410). Things in general are thought not in relation to the conditions of experience but only as they are in themselves. Leaving out all sensible conditions, any thinkable determination of them would thus be purely intellectual, and the objects would be intelligible objects. We may call these intelligible things in themselves intelligibilia.Footnote ²⁶

We can find the assumption of intelligibilia as related to the idea of God in the introduction to the Dialectic:

The ens realissimum is not construed through direct reference to objects of experience, in contrast with the cosmological idea which is construed from the totality of appearances. Rather, it is inferred from the ‘conditions for the possibility of things in general’, hence things-in-themselves with which we are not acquainted. Although nothing can be known about such things, reason assumes their existence as purely intelligible when inferring the existence of God.

To get a clearer view of the characteristics of these objects, we should note that the ens realissimum is not only the ground of thoroughgoing determination and the possibility of everything else, it is also described as the prototype, the perfect example, of things in general: ‘the ideal is thus the original image (prototypon) of all things, which all together, as defective copies (ectypa), take from it the matter for their possibility’ (A578/B606)

The Platonic overtones in the above passage are not accidental. At the outset of the Dialectic Kant explicitly borrows the term ‘idea’ (Idee) from Plato to designate concepts of objects that cannot be given in experience but serve as standards of perfection (A320/B377). Most of Kant’s examples for the significance of Platonic ideas come from the practical realm. But the Ideal chapter shows how theoretical considerations also lead reason to form an ontology of degrees of reality so that the ens realissimum grounds the possibility of all its less perfect copies. By conceiving the possibility of things in general as intelligibilia, reason reifies concepts as objects and makes them dependent on the most perfect object which corresponds with the most complete concept. This is, in other words, Platonism.

This Platonic picture entails that, by discerning the features of the prototype, i.e. the ens realissimum, we can learn what is assumed about the copies, i.e. the objects which depend on it. Because the ground of thoroughgoing determination is also the ground of possibility, this also clarifies the ontological commitments of the possibility proof.

The ens realissimum is a paradigmatic example of a thing-in-itself because by having all perfections and no negations it is individuated solely by conceptual means:

The Amphiboly chapter provides further details about the characteristics of things-in-themselves as intelligibilia. The target of criticism in this chapter is the Leibnizian theory of monads, and the basic thought is that this theory could be justified according to the concepts of reflection if we had cognition of intelligibilia. Yet we do not possess such cognition and the principles valid for intelligibilia are not valid for objects of sensible intuition. What is important for the present purpose is that we can also learn from this discussion what intelligibilia would be like had they existed, and why they are presupposed in the construal of the ens realissimum.

The first pair of concepts of reflection states the identity of indiscernibles (A263–4/B319–20). As noted in section 2, the principle of thoroughgoing determination as a principle of individuation depends on this principle, and the ens realissimum itself as a concept of an individual exemplifies it. Thus the principle of thoroughgoing determination from which the construal of the ens realissimum begins is fully applicable only to intelligibilia. Also according to the second and the third pair of concepts of reflection, the ens realissimum is a paradigmatic example of an intelligible thing in itself: there is no opposition between its realities, and all its determinations are intrinsic rather than relational (A264–6/B320–2).

The Amphiboly chapter does not discuss concepts of modality, but the last pair of concepts of reflection, matter and form, is directly related to the grounding of possibility:

Appearances are first individuated by the spatiotemporal form, and only then determined with regard to their content: ‘the form of intuition (as a subjective constitution of sensibility) precedes all matter (the sensations), thus space and time precede all appearances and all data of appearances, and instead first make the latter possible’ (A267/B323). But regarding intelligibilia, if there were such things, the existence of the matter of their reality could be inferred from their real possibility. The affinity of this discussion of matter and form in the Amphiboly to the construal of the ens realissimum and the possibility proof is clear. The reasoning to the matter of all possibility is conditionally valid for intelligibilia: if one could cognize the existence of such objects, then one could also cognize a sum total of all intelligible reality grounding their possibility.

We should also notice in the above passage that components of intelligible objects are modelled after concepts (the predicates being related in judgement) and are called essentialia, the material to be combined in the essential form of a thing. When considering intelligibilia, the matter of all possibility is the totality of essences. Thus the conclusion of the possibility proof that the matter of all possibility is grounded in God is equivalent to the grounding of all essences in God. This notion of a unified ground of essences is exactly what Kant found valuable in the conception of God entailed by the possibility proof and allows its transformation into a regulative idea, as I will suggest in the next section.

To conclude: since the ens realissimum as the paradigmatic concept of an intelligible thing-in-itself is inferred from being the archetype (‘the original image’) grounding the possibility of all other things, those things, i.e. intelligibilia, are assumed to exist.

7. The Regulative Use of the Ideal

In the series of the metaphysical errors following the formation of the ens realissimum, it is not only realized and hypostasized as an existing object but also made into a thinking substance:

As argued above, this whole series of errors is motivated by an interest in collective unity. Since this is a conceptual rather than sensible unity, it is plausible to regard its ground as intelligent. But what is important for the present purpose is that this unity is described as regulative, hinting at the positive regulative role of the ens realissimum.

The regulative use of the ideas of reason is the subject of the Appendix to the Dialectic. The idea of God is specifically related to the systematic unity of nature: ‘the idea of that being [God], means nothing more than that reason bids us consider every connection in the world according to principles of a systematic unity’ (A686/B714).

In contrast with the legitimate regulative use, the possibility proof not only ‘bids us consider’, but concludes that there exists an object grounding the systematic unity of all possible things. But leaving aside the objective status of the conclusion, it is important to note that in the pre-Critical possibility proof the conception of God is also related to the systematic unity of nature.

The second part of OPA (2: 93–155) discusses at length the systematic order of nature. Kant there criticizes explanations of natural facts that appeal to contingent divine intervention instead of necessary laws of nature. But investigating the latter is not in contrast with including God in the metaphysical explanation of nature. The ground of all possibility is the ground of essences, i.e. the necessary properties of things and the lawful relations between them (cf. Insole Reference Insole2011). Hence investigating the unity of the laws of nature stemming from the essences of things coheres with the conception of God as the single ground of all possibility established in the first part of OPA:

Kant espoused this view about God as the ground of essences and the laws of nature at least from the 1750s in his drafts on optimism (R3704, 17: 234) and the Universal Natural History (1: 222–3, 332), and he continued to hold it in LPR (28: 1035).

The usefulness of this conception of God is transformed in the Critique into a regulative idea expressing the need to assume the unity of the laws of nature.Footnote ²⁷ But taking this need as objective rather than subjective leads to the dogmatic assumption of the possibility proof, that essences exist as intelligible things in themselves from which it is possible to infer their ultimate ground, God.

In LPR Kant contends that the possibility proof establishes ‘the subjective necessity of assuming such a being’ for explaining ‘what in general the possibility of something consists in’ (LPR, 28: 1034). A note from the 1770s provides further elaboration of this subjective necessity:

The possibility proof is invalid without the assumption of a priori knowledge about the conditions of possibility in general. Since it proceeds from the ontological ground of possibility in general and not from the epistemic condition for cognizing the possibility of empirical objects, such a priori knowledge would amount to cognition of the matter of possibility as intelligible, hence knowledge about the existence of intelligibilia. Footnote ²⁸ Assuming this kind of knowledge is, in fact, assuming that the structure of thought is also the structure of reality,Footnote ²⁹ and therefore the argument is subjectively valid for human thought, cat anthropon (ad hominem). While this subjective validity is the illusion motivating the unjustified assumptions of the possibility proof, it also correlates with its positive regulative use. Thus although there can be no cognition of essences qua intelligibilia, the idea of God construed as their ground expresses the rational interest in discovering the unity of the laws of nature.