By Amogha Sahu, University of Toronto
In this paper, I will analyze Kant’s concept of the ‘thing-in-itself.’ I will begin by providing a summary of Kant’s metaphysics leading into his discussion of the ‘thing-in-itself’, at the end of the Transcendental Analytic. I will then provide a summary of Kant’s account of the ‘thing-in-itself’ and how it relates to his metaphysics in the chapter “On the Essence of the Distinction between Phenomena and Noumena.” I will argue for an account of the thing-in-itself as a regulative idea, with an account of how this might relate to Kant’s practical philosophy in the Critique. Along the way, I will discuss various interpretations of the thing-in-itself present in the Kant literature.
- Kant’s Transcendental Idealism: A Summary
In the Critique of Pure Reason, Kant aims to provide a systematic basis for the possibility of metaphysics. He wishes to do this by demonstrating the possibility of synthetic a priori judgements . In order to understand why Kant thinks that demonstrating the possibility of synthetic a priori judgements would lead to a systematic basis for metaphysics, it is necessary to understand what synthetic a priori judgements are. Firstly, Kant thinks that synthetic judgements are judgements where the predicate is not contained in the concept of the subject . He counterposes these judgements to analytic judgments, where the predicate is contained in the concept of the subject. For Kant, these amount to definitions, or explications of the subject, and as such, are merely the product of conceptual analysis, with no direct application to, or input from, experience. Thus, they have no direct metaphysical application, as they have no content, and can tell us very little about the world that we experience. For example, the judgement ‘All bachelors are unmarried’ provides no knowledge about the world, as the predicate ‘unmarried’ is already contained in the concept of the subject, ‘bachelor.’ Therefore, it is an analytic judgement. Synthetic judgements, on the other hand, are not merely tautological definitions. They are supposed to be a source of knowledge about the world. For example, the judgement <every alteration has a cause> is genuinely informative, as the concept of alteration does not contain the concept of cause (according to Kant).
Secondly, Kant thinks that a priori judgements are judgements of the intellect which may arise from, but do not depend for their validity on, experience. For example, <All bodies are divisible> cannot be falsified or verified based on our experience of bodies. It is simply the result of the conceptual analysis of the concept of body and the concept of divisibility, which demonstrates that the concept of divisibility is contained in the concept of body. Kant also conceives of a priori judgements as being universal and necessary, as they do not rest on particular, contingent experiences of the world. The validity of a posteriori judgements, on the other hand, have a particular, contingent, sensory basis.
Thus, Kant’s problem is as follows: How is it that we can have knowledge of necessary truths, providing what he sees as a ‘sound basis’ for our knowledge, given that (i) all knowledge about the world comes from sensory experience (the so-called synthetic a posteriori judgements), which is informative but contingent (and thus cannot provide a universal and necessary foundation for our knowledge) and (ii) all necessary truths seem to have only a formal validity (bounded only by the law of non-contradiction) and thus do not provide us any knowledge about the world (analytic a priori judgements, which possess necessity but are not informative)? In other words, Kant is looking for judgements which have necessity and universality (and are thus a priori) but are also a source of non-trivial or non-tautological knowledge about the world (and are thus synthetic). The way that Kant bridges the contingency of the synthetic a posteriori judgements and the merely formal or logical validity of the analytic a priori judgements (thus making way for the synthetic a priori) is with his notion of the conditions of possibility of experience. This notion allows Kant to integrate two propositions, that (i) only sensory experience can provide contentful knowledge and (ii) the necessary truths produced by the intellect (or, more accurately, Kant’s pure concepts of the understanding) can only have formal validity. The way this integration takes place is through Kant’s claim that experience has both a form and a content. Although sense-perception provides the content of experience, pure concepts of the understanding have what he calls a transcendental role, in that they provide the formal structure for experience. This formal structure has what Kant thinks is a ‘conditioning’ function, in that it grounds or makes possible what we consider to be our everyday, unmediated, experience. Kant sees the elaboration of this necessary structure of experience as being a source of synthetic a priori judgements, providing a new ‘transcendental metaphysics of experience.’
The ‘elaboration’ of this formal structure is what occupies Kant throughout the rest of the ‘Transcendental Analytic’ of the Critique of Pure Reason. The ‘Analytic’ is divided into two sections: The Transcendental Logic and the Transcendental Aesthetic. The Transcendental Aesthetic is preoccupied with the problem of the form of sensible intuition. Kant distinguishes between the faculty of sensible intuition and the faculty of understanding. For Kant, intuition refers to our ability to immediately receive sensible data from the world around us, while the understanding refers to our ability to synthesize and unite this manifold of indeterminate sensible data through conceptual judgements.
In the Aesthetic, Kant wants to (methodologically) jettison “what the understanding mediately thinks through the concepts” and wants to investigate the form in which we immediately receive sensible data. For Kant, sense-data is always received in a spatio-temporal form. That is to say, we necessarily represent the objects outside us as existing in a particular place and at a particular time. In his analysis of sensible intuition, Kant distinguishes between outer sense and inner sense. He sees spatiality as being outside the subject of experience, and thus sees spatial objects as objects of outer sense. Space is thus considered to be the ‘form of outer sense.’ Temporality, on the other hand, is internal to our representations (the form of ‘inner sense’). Our perceptions have a temporal sequence and by virtue of their temporal nature, they provide ‘temporality’ to the outer, spatial, objects.
In the Logic, on the other hand, Kant wants to analyze the faculty of understanding, which is better understood as the ‘faculty of conceptual judgements.’ Kant conceives of concepts as ‘possible predicates of judgements.’8As the spatio-temporal data from sensible intuition is indeterminate and diffuse, it has to be synthesized through the faculty of understanding (to produce a ‘unified’ manifold)9and then structured through the application of concepts through conceptual judgements. At this point, it is important to cite Kant’s distinction between pure concepts and empirical concepts. Empirical concepts, which Kant sees as being the province of the English empiricists, Locke and Hume, are ‘abstract generalizations’ from many particular, concrete instances of sense-experience. He sees these as contingent, dependent on particular, subjective experience. What he is really concerned with when he speaks of conceptual judgements ‘structuring’ experience are ‘pure concepts,’ such as substance, or cause, or unity, which are not ‘abstractions’ from sense-experience.
Kant believes that pure concepts can be traced back to a set of ‘ancestral concepts’ called the ‘categories’ (which include ‘substance’ and ‘cause’), which he argues can be derived from the ‘form of judgement’ itself. Kant spends a great deal of time in the ‘Transcendental Deduction’ of the Logic trying to demonstrate that, by virtue of the fact that the categories can be derived from the form of judgement, that the categories have ‘objective validity’; that these ‘pure concepts’ provide a form for objects of experience. He does this by demonstrating that our experience has a ‘mineness’; that there exists a ‘subject’ (at a formal level) which ‘has’ experience. This ‘subject’ of experience, which Kant calls the ‘transcendental unity of apperception, is ‘presented’ with a ‘unity of representation’ (which can be thought of as an ‘object’ of experience ‘synthesized’ by the productive imagination). Kant sees this ‘subject-object’ structure as being a condition of possibility of all experience, as a ‘form’ which structures experience.
Furthermore, he sees the ‘deduction’ of this ‘subject-object’ structure as a demonstration that the ‘form of judgement’ (which is structured in terms of subject and predicate) is a condition for the possibility of experience. In demonstrating that the ‘form of judgement’ plays this conditioning role, Kant also thinks that he has demonstrated the ‘objective validity’ of the categories, and their application to experience (through categorical judgements). The manifold of intuition (the ‘object’) synthesized under the transcendental unity of apperception (the ‘subject’) allows for the imposition of certain ‘rules’ (through conceptual judgement) on the indeterminate mass of intuition. These ‘rules’ provide what Kant calls the ‘formal conditions of experience,’ normative rules for ‘correct’ experience which allow us to distinguish between a veridical and an illusory perception.
The way that Kant sees this ‘application to experience’ occurring is through what he calls the ‘schematizing’ function of the imagination (part of the faculty of understanding). The ‘Schema’ connects the pure concepts of the understanding and sensory experience through time. As Kant puts it, “The schema of a pure concept of the understanding, is something that can never be brought to the image at all, but is rather only the pure synthesis, in accord with a rule of unity according to concepts in general.” That is to say, each pure concept possesses a schema, which should be understood as a set of rules to synthesize the manifold under a pure concept, which allows the concept to be applied to our faculty of sensible intuition.
The connection between intuition and the understanding occurs when Kant demonstrates the connection between the schema and the form of inner sense (or time). For Kant, the connection between particular, contingent objects and the categories is that particular, contingent objects are perceived in time, and as such, they appear at a particular point in a universal time-series. Thus, the categories can be applied to contingent objects of experience, when, through the schema, a connection has been demonstrated between the category and the universal time-series, such that the categories become rules which govern the universal time-series.
An example of this ‘application of the categories to experience’ is the category of causality. In the Second Analogy of Experience in the Transcendental Analytic, Kant demonstrates the way that the categorical judgement that <every alteration has a cause> structures experience. Given that the schema of causality dictates that every event in a time-series follows every other in a rule-governed succession, this acts as a ‘form’ in which we process events in the temporal sequence of everyday life. The category of cause conditions how we necessarily perceive everyday events, as well as how we engage with the world. For example, as I type this essay, two events recur, which I take to be causally connected. Firstly, I punch a key on the keyboard. Secondly, a letter appears on the screen. By virtue of the category of cause, those events appear to me as a necessarily connected causal flow, and it is only in reflection that I separate them and find them without a causal relation.
Thus, by the end of the systematic representation of the principles of pure understanding, Kant has given us a comprehensive picture of his ‘transcendental idealist’ metaphysics. As concept-using, spatio-temporal beings, the world that discloses itself to us in experience is a spatio-temporal world governed by certain rules (the categories). This does not entail, as some readers of Kant suggest, that the objects of experience are mere chimeras generated by us, as this entails a confusion between form and content. We, by virtue of the limitations of the ‘human standpoint,’ might contribute the form in which we experience outer objects, but we do not contribute the objects themselves. Thus, the objects of experience are not mental in nature.
- The Thing-in-Itself
Kant introduces the thing-in-itself when he speaks about the mistakes of previous metaphysicians, both in the introduction to the Critique and in the Transcendental Aesthetic. For Kant, traditional metaphysics (particularly what he calls the dogmatic metaphysics of Leibniz or Wolff) assumes a problematic pre-ordained harmony between subject and object. It suggests that we have a direct access to the objects of experience in themselves, without any conditioning work from our own mental faculties. That is, the objects of experience provide their own form and content, thus grounding themselves. Kant calls this position transcendental realism. Kant rejects this position, as the entirety of the Transcendental Doctrine of Elements (both the Aesthetic and Logic) is intended to demonstrate how what we assume is unconditioned access to outer objects in experience is actually conditioned by a host of mental faculties, which provide conditions of possibility for experience. From the Transcendental Doctrine of Elements, Kant derives two arguments against transcendental realism.
Firstly, the transcendental realist assumes an unproblematic harmony between subject and object, and is unable to account for why this harmony exists without invoking a benevolent God who designed human faculties so that they would provide direct, unmediated access to objects outside us. Secondly, Kant undercuts the dogmatic proof for God by claiming that the traditional proofs of God, which rely on the extension of concepts such as cause and substance over and above the bounds of possible experience (as God is supernatural in dogmatic conceptions). Kant’s objection to these arguments for God is based on his notion that concepts only have empirical validity, they only have validity when applied to the objects of experience. For Kant, concepts such as cause and substance structure our experience, and using concepts as if they provided their own objects independent of sensory experience (for example, the notion that there is such a thing as a substance, out there, independently of our faculties) is illegitimate.
This is where Kant’s notion of the thing-in-itself comes in. Given his transcendental idealism, Kant sees the objects of experience as being ‘conditioned’ by the concepts of the understanding and the form of sensible intuition. However, Kant claims that, given the conditioned nature of our knowledge, we can have no empirical knowledge about unconditioned things-in-themselves. However, he seems to suggest that we have some kind of knowledge about things-in-themselves.
Firstly, the thing-in-itself is a problematic concept. It is a ‘thing’ which is not in space or in time. It is also not bound by any of the rules of causality, substance, community or modality which regulate the objects of experience. This is because “concepts are entirely impossible, and cannot have any significance, when an object is not given for them” and “the modification of our sensibility is the only way in which objects are given to us.” Thus, concepts “cannot pertain to things in themselves.” As Michelle Grier points out, the application of the categories beyond sensibility, to things-in-themselves leads to the erroneous conclusion that the “logical function of judgements” can be seen as the “condition of the possibility of the things themselves.” This is equivalent to the notion that the categories can produce their own ‘intellectual object,’ and thus can provide the condition of possibility for those objects independently of the faculty of sensible intuition.
Secondly, he suggests that we can think, but not cognize things-in-themselves. For Kant, cognition is bounded both by sensible data and by the conditions of possibility provided by the faculty of sensible intuition and the faculty of the understanding. Thus, we can only have cognitions about objects of experience. Thought, on the other hand, is only bounded by the principle of non-contradiction, and can thus be the source of any number of outlandish ideas. It can thus be the source of an idea (the thing-in-itself), which, (even though it is so removed from our everyday experience that it is difficult to picture or even conceive of it), does not violate the principle of non-contradiction. However, this prevents any possibility of contentful, non-trivial knowledge about things-in-themselves. So what role does the thing-in-itself play? Is it simply a figment of Kant’s imagination intended to ridicule the transcendental realism of dogmatic metaphysics? To answer this question, it is necessary to investigate Kant’s account of the distinction between phenomena and noumena and its relation to the problem of the thing-in-itself.
- Noumena and Things-in-Themselves
In the Preface to the Critique, Kant makes a distinction between phenomena (the world of experience) and noumena (the world of the intellect). The phenomena is the realm of appearances, governed by the forms of sensible intuition and the categories, while the noumena is described by Kant as the “realm of the intellect,” where the understanding is not provided an object by sensible intuition, but by the intellect itself. This is slightly paradoxical, considering that Kant states earlier that the understanding can only be provided an object of sensible intuition.
However, Kant insists that “the word ‘appearance’ must already indicate a relation to something, the immediate representation of which is, to be sure, sensible, but which in itself, without this constitution of our sensibility, must be something, i.e. an object independent of sensibility.” This reference to the thing-in-itself as being a separate object in a different realm (the noumena “conceived in the positive sense,” as Kant calls it). This has led to some interpreters (most prominently, P.F. Strawson) speaking of “two worlds” in Kant’s metaphysics. Strawson speaks of the “supersensible realm” or noumena populated by things-in-themselves. However, this interpretation immediately runs aground, for two reasons. Firstly, as Lucy Allais points out, ‘two-world’ interpretations are committed to what Kant calls noumena in the positive sense. Kant rejects this claiming that the “division of objects into phaenomena and noumena” can “therefore not be permitted at all <in a positive sense>.” Secondly, I refer to the paradox presented earlier, where Kant seems to be committed to special ‘intellectual objects or objects of non-sensible intuition’ in the noumenal realm, but he also suggests that concepts can only be provided their objects by the understanding. However, this paradox is dissolved if we repudiate this ‘two-world’ conception of Kant’s transcendental idealism.
Another problem arises when we consider Kant’s suggestions that the thing-in-itself is simply the object ‘considered’ in another way. This is the “double-aspect” theory of the distinction between objects of appearance and things in themselves. In this view, there are ‘phenomenal’ characteristics of the object which are disclosed when we represent objects as existing in space and time and synthesize the manifold according to the categories. However, if there is a mode of representation which is different from the “human standpoint,” it discloses some ‘noumenal’ characteristics of the same object. Thus, the same object has two aspects, ‘the phenomenal aspect’ and the ‘noumenal aspect.’ This view denies that there are ‘two worlds’ or that phenomena are mental, but that there are two different ‘ways’ of representing objects.
This interpretation provides a useful account of Kant’s compatibilism, by allowing for a ‘phenomenal’ self, governed by natural causes, and a noumenal self (somehow existing outside of time) which has transcendental freedom. However, it is not without its problems. Suggesting that the same “object” can be interpreted in two ways is misleading, as it presupposes that the same object can be individuated when the categories, and their application to experience, are removed. It is unclear that one can refer to <the table> having <noumenal properties>, given that, in order to individuate the table as table, you need the categories (particularly unity, totality, and substance). However, if we interpret the word <object> in a less restrictive manner, and consider it simply as something our thought is directed to, it may be less problematic. Nick Stang makes a similar point when he emphasizes that speaking of a noumenal aspect to the object, given that the ‘noumenal aspect’ has completely different properties from the phenomenal aspect, actually entails giving up on the identity of the object between it’s phenomenal and noumenal characteristics. Thus, the dual-aspect interpretation, may, on some readings, risk falling back into the two-world interpretation. It furthermore, suggesting that the ‘thing-in-itself’ is a different mode of representation, seems to me to hint at making some commitments as to what it is. I think that this is risky, given that Kant seems to emphasize that the thing-in-itself should be understood negatively.
It is also important not to confuse the thing-in-itself and the transcendental object. The transcendental object is the concept of the object in general, which is part of the conditions of possibility for experience. It is a formal concept governed by the categories, unlike the thing-in-itself. Michelle Grier sometimes seems to hint that the two are more similar than I suggest when she claims that the “concept of an object in general” (which is an abstraction) is equivalent to the notion of the thing-in-itself. While it is true that Kant refers to the thing-in-itself as an abstraction, an intelligible object or nonempirical object, it is important to distinguish between the ‘transcendental object’ or the ‘concept of the object in general’ as used in the transcendental deduction and the ‘transcendental object’ as used by Kant with respect to the thing-in-itself. The thing-in-itself is an abstract transcendental object in an important sense, in that it is a result of thought, but it results from the transcendental use of the categories, while the transcendental object is part of the empirical use of the categories, in that it acts the condition of possibility of experience.
Another difficulty which arises in the consideration of the thing-in-itself as being metaphysically separate is the problem of affection. Given that we cannot know anything about the thing-in-itself (particularly, we cannot apply the category of cause to it), it is unclear that Kant can reconcile this with the notion that the thing-in-itself affects us causally, especially when he wants to give an account of how an uncaused ‘noumenal self’ having freedom to act in the world. Given that we have an account of what the thing-in-itself is not, what is the thing-in-itself?
- The Thing-in-Itself as a Regulative Idea
Kant seems to continually suggest that the noumena is a “limit concept.” This is the concept of noumena in the negative sense which Kant seeks to endorse, given his misgivings about the noumena in the positive sense. In this conception, the noumena is to be considered as the limit or boundary of our possible experience. Considering that the noumena is the ‘world of thought’ and the thing-in-itself is a thought-form, it is not inconceivable that the thing-in-itself might have some sort of negative role, as opposed to the positive role which both the ‘two-world’ and the ‘double-aspect’ interpretations. Furthermore, emphasizing the thing-in-itself’s negative role helps to make sense of its inconceivability and to minimize the problems that occur whenever anyone wants to give some kind of positive account of the thing-in-itself, such as the problem of affection.
One way to talk about of the thing-in-itself negatively, as well as speaking about the role it plays in Kant’s system in terms of restricting the extravagant claims of dogmatic metaphysics is to speak of it as a regulative idea of reason. Kant distinguishes between constitutive principles and regulative principles. Constitutive principles, such as the principle of causality discussed in the Analogies, lay down the conditions of possibility for experience and ‘constitute’ the character of experience by doing so. Regulative principles, on the other hand, help us correct errors in thinking, preventing the understanding from ‘soaring beyond the bounds of possible experience.’
For Kant, reason always seeks the ‘unconditioned,’ an ‘ungrounded ground’ which provides a complete foundation for all objects of experience. It is this impulse which leads to the postulation of certain entities, such as God, the world as a whole, or the soul, which provide a unity and a totality to our concepts of the understanding as they help to ground the objects of experience. However, Kant insists that we cannot really have any knowledge beyond the bounds of possible experience. Thus, these ideas cannot be considered to ‘apply’ to or ‘ground’ objects of experience (as the dogmatists might suggest) and as such must have only a regulative use. They cannot refer to experience positively, but can only do so indirectly, by ordering the concepts of the understanding such that they do not move beyond the bounds of possible experience. They are ‘heuristics’ or tools to guide our use of reason and to prevent us from making errors. Thus, although reason is restricted by possible experience, and is thus is unable to provide a theoretical unity to the concepts of the understanding (it has to restrict them to their empirical use, and prevent their transcendental use, even though reason seeks to have knowledge of things-in-themselves), it can accomplish a practical unity, by ‘filling the gaps’ left by Kant’s claim of noumenal ignorance through the use of Ideas like God, the soul and the world as practical principles, which help to orient our action, but cannot be a true source of knowledge. I claim that the thing-in-itself plays a similar role in Kant’s system. I have already claimed (above) that the thing-in-itself is better understood negatively, as a boundary concept which helps to define the conditions of possibility for experience, rather than as a positive claim about a different ‘metaphysical reality’ from the objects of experience. The regulative use of the thing-in-itself can be demonstrated by the fact that it is essentially a heuristic, a tool which is useful for guiding our thought which cannot itself provide us with concrete knowledge about the objects of experience.
Michelle Grier draws a distinction between the thing-in-itself as a negative concept (in the sense endorsed above) and the positive use of regulative ideas such as the world for the practice of natural science. Ideas of Reason like God, the world as a whole (Nature) and the soul (possessing free will, and thus, moral responsibility) can be conceived positively, and although they have no validity with respect to the objects of experience, they can work as a set of heuristics which help to guide our scientific (in the case of the world as a whole) and moral (in the case of God and the immortal soul) inquiry. While it is the case that the thing-in-itself cannot really be conceived other than negatively (which distinguishes it from the three regulative ideas shown above), it can play a regulative role in so far as it possesses two characteristics. Firstly, it is a rule which has no empirical validity (unlike the constitutive rules of the categories). This is the case, as the thing-in-itself is completely distinct from the objects of experience. Secondly, nonwithstanding its lack of empirical validity, it plays a heuristic role in our inquiry. This is the case for the thing-in-itself (considered negatively), in so far as it prevents the transcendental use of the categories or the concepts of the pure understanding. Thirdly, Kant insists that the thing-in-itself is thinkable without contradiction, just as the other three regulative ideas are.
Although some may object that the thing-in-itself has no role in terms of helping to orient our practical reason and is simply a metaphysical notion, it is possible to think of Kant’s conception of the thing-in-itself as a contribution to his general goal of preserving human autonomy. Dogmatic metaphysics claims to ‘overcome’ what Kant sees as the limitations of the human standpoint by claiming that we can access the thing-in-itself through our reason. In doing this, certain dogmatic systems (particularly that of Leibniz) have also attempted to override the autonomy of rational human beings through their determinism. In so far as the thing-in-itself in the negative sense prevents the reformulation of these deterministic systems, leaving a margin of freedom for human beings, it can play an important regulative role as a principle of practical reason. Thus, this view also strengthens the connection between Kant’s theoretical project and his practical project, as it connects his ‘prohibition’ to his practical philosophy. Most importantly, it prevents readings of Kant which seem to see in him a new dogmatic metaphysics by stealth, by reading the thing-in-itself as another realm or a different object, rather than as a simple ‘boundary’, limiting the conceivable from the inconceivable, an important representative of Kant’s project in general, the imminent critique of reason, limiting itself through its own operation.
- Ameriks, Karl. Interpreting Kant’s Critiques: Kant and traditional ontology; Kant and Short Arguments to Humility. Oxford University Press, 2003.
- Grier, Michelle. Kant’s Doctrine of Transcendental Illusion. Cambridge University Press, 2001.
- Banham, Gary. “Regulative Principles and Regulative Ideas.” (2013).
- Stang, Nicholas F. “The Non Identity of Appearances and Things in Themselves.” Noûs 48, no. 1 (2014): 106-136.
- Matthews, H. E. “Strawson on Transcendental Idealism.” The Philosophical Quarterly (1969): 204-220.
- Strawson, P. “The Bounds of Sense: An Essay on Kant’s Critique of Pure Reason, Methuen & Co.” Ltd., London (1966).
- Allais, Lucy. “Kant’s One World: Interpreting ‘Transcendental Idealism’.” British Journal for the History of Philosophy 12, no. 4 (2004): 655-684.
- Gardner, Sebastian. Routledge Philosophy Guidebook to Kant and the Critique of Pure Reason. Psychology Press, 1999.
- Kant, Immanuel, and Paul Guyer. Critique of Pure Reason. Cambridge University Press, 1998.
 Kant, Immanuel, and Paul Guyer. Critique of Pure Reason. Cambridge University Press, 1998. P.338
 CPR p.146 B19.
 CPR p.141 A7/B11.
 CPR p.137 B3
 CPR p.150 B27.
 CPR p.150 B30
 CPR p.157 A22
 CPR p.213 A82/B108.
 CPR p.205 B93.
 He calls the deduction of these empirical concepts an “empirical” deduction, while he wants to undertake a transcendental deduction. CPR p.220 A85/B117.
 CPR p. 213 A81/B107.
 CPR p.250 B139.
 CPR p.252 B143.
 CPR p.271 A142.
 CPR p.272 A138/B177.
 CPR p.304 B233.
 CPR p.551.
 Kant speaks about “everything in our cognition that belongs to intuition” contains nothing but “mere relations,” and that “through mere relations no thing in itself is cognized.” CPR p.189 B67
 CPR p. 272 A139/B178
 Grier, Michelle. Kant’s Doctrine of Transcendental Illusion. Cambridge University Press, 2001. p.66.
 CPR 115.
 CPR p.115 Bxxvi/Bxxvii.
 CPR p.347 A249.
 CPR p. 348 A252.
 . Strawson, P. “The Bounds of Sense: An Essay on Kant’s Critique of Pure Reason, Methuen & Co.” Ltd., London (1966). p.236.
 Allais, Lucy. “Kant’s one world: Interpreting ‘transcendental idealism’.” British Journal for the History of Philosophy 12, no. 4 (2004): 655-684. p.658.
 CPR p.362 B31.
 Matthews, H. E. “Strawson on transcendental idealism.” The Philosophical Quarterly (1969): 204-220.
 CPR p.536 a539/b567.
 Stang, Nicholas F. “The Non-Identity of Appearances and Things in Themselves.” Noûs 48, no. 1 (2014): 106-136. Grier, Michelle. Kant’s Doctrine of Transcendental Illusion. Cambridge University Press, 2001. p.80.
 Kant sometimes does seem to suggest that there might be a different form of intuition, intellectual intuition, not possessed by us, but which might provide access to the things-in-themselves. However, he stresses that this hypothesis is speculative, at best. As he puts it, “it even remains unknown whether such a transcendental (extraordinary) cognition is possible at all” (p.364 B314) and “we have no insight into the possibility of such noumena” (CPR p.362 a255).
 Sometimes Kant seems to use the two interchangeably, especially on p.536 A539/B567.
 Grier, Michelle. Kant’s Doctrine of Transcendental Illusion. Cambridge University Press, 2001. p.80
 We can think of the ‘transcendental object’ as an abstract image, conceptualizing what it is for something to be an object for us. As such, it is an image of an object which stands under the categories, and as such, it acts as a formal structure within which we can cognize the objects provided us by the faculty of sensivle intution.
 CPR p.538 A540/B568
 CPR p.363 A256.
 Banham, Gary. “Regulative Principles and Regulative Ideas.” (2013).
 CPR p.461 B436.
 CPR p.520 A508/B536.
 CPR p.672 A796/B824.
 Grier, Michelle. Kant’s Doctrine of Transcendental Illusion. Cambridge University Press, 2001. P.266.
 Karl Ameriks points out that Kant sees his transcendental arguments as trying to demonstrate human autonomy. Ameriks, Karl. Interpreting Kant’s Critiques: Kant and Traditional Ontology; Kant and Short Arguments to Humility. Oxford University Press, 2003. P.5
 CPR p. 101 Axi.