Tuesday, 25 June 2013

Kant's Argument for Transcendental Idealism

Kant's Transcendental Idealism is the metaphysical view which he states as follows:
... all objects of a possible experience ... have no self-subsistent existence apart from human thought.
Time and space, with all phenomena therein ... cannot exist out of and apart from the mind.
I formulate this as:
Time and space, and all objects of a possible experience, cannot exist out of and apart from the mind.
How does Kant arrive at this conclusion, that space, and time, and rocks, and trees, and quasars and so on, "cannot exist out of and apart from the mind"?

Here I quote from my electronic copy of Kant, Critique of Pure Reason (1781/1787) (tr., J. M. D. Meiklejohn). (If you find Kant's organization of CPR very confusing and Byzantine, don't worry: so, do I. Here it is online.) I highlight in bold what seem to be the central claims:
Transcendental Logic, Second Division: Transcendental Dialectic
BOOK II: The Dialectical Inferences Of Pure Reason
SECTION VI. Transcendental Idealism as the Key to the Solution of Pure Cosmological Dialectic.
In the transcendental aesthetic we proved that everything intuited in space and time, all objects of a possible experience, are nothing but phenomena, that is, mere representations; and that these, as presented to us—-as extended bodies, or as series of changes—-have no self-subsistent existence apart from human thought. This doctrine I call Transcendental Idealism. ...
Transcendental idealism allows that the objects of external intuition—-as intuited in space, and all changes in time-—as represented by the internal sense, are real. For, as space is the form of that intuition which we call external, and, without objects in space, no empirical representation could be given us, we can and ought to regard extended bodies in it as real. The case is the same with representations in time. But time and space, with all phenomena therein, are not in themselves things. They are nothing but representations and cannot exist out of and apart from the mind.
So, if I follow Kant correctly, here is Kant's argument for (TI):
Assumption 1: Space is the form of external intuition.
Assumption 2: Space (and time) is necessary for the representation of objects (of a possible experience).
Assumption 3: External intuition is a property of the mind.

Claim (TI):
Space, and all objects of a possible experience, cannot exist out of and apart from the mind.
[I ignore the time part of the claim, as it simply seems to make the argument more complicated without adding anything new.]

Proof: By Assumption 1, space is the form of external intuition. But, by Assumption 3, external intuition, being a property of mind, cannot exist independently of human thought. Thus space cannot exist out of and apart from the mind. By Assumption 2, space (and time) is necessary for the representation of objects. So, objects of a possible experience cannot exist independently of external intuition, and, a fortiori, cannot exist out of and apart from the mind. QED.

This argument is not a precise, valid, formal argument. Still, it seems as close to being informally valid as one might reasonably request of a philosophical argument.

[UPDATE: 1 July. I've separated out the two important quotes at the start, and formulated (TI) based on both.]


  1. That seems more or less OK. Time is necessary to make the conclusion universal, since not all mental representations (e.g. beliefs, concepts, etc) are spatial, but all of them are temporal. In particular, you need time in order to get transcendental idealism about the self: the self as it seems to itself is mere phenomenon.

  2. Many thanks, Brian,
    (Just focusing on Kant's main argument for the (TI) claim that space and objects of experience are incapable of existing apart from the mind.)



  3. Kant says, at the beginning of the passage you quote, that he proved Transcendental Idealism in the Transcendental Aesthetic. Why are you then looking for an argument for Transcendental Idealism in this passage from the Transcendental Dialectic? It doesn't seem to me that he is making any such argument there, just restating some conclusions that he thinks are already established as part of showing how they are supposed to resolve the antinomy.

    (Michael Kremer)

  4. Many thanks, Michael.

    That's right, yes.
    But do you think the argument he had given earlier is different from the argument he gives in the quoted passage? In the quoted passage, he is summarizing the earlier argument for (TI), based on the assumption that space is the form of external intuition.



  5. Jeffrey,
    Your question to me presupposes that in the quoted passage he is giving an argument for Transcendental Idealism (then I can say whether this argument is the same as or different than the one in the Aesthetic). I reject the presupposition.

    The quoted passage in fact does contain one argument (indicated by the word "for") but this is an argument for "empirical realism," that is for the claim that (in spite of Transcendental Idealism) "the objects of external intuition—-as intuited in space, and all changes in time-—as represented by the internal sense, are real." That this is Kant's point is made clear by the parts you left out from the quoted passage. Immediately before your quoted second paragraph, Kant says "It would be unjust to accuse us of holding the long-decried theory of empirical idealism, which, while admitting the reality of space, denies, or at least doubts, the existence of bodies extended in it, and thus leaves us without a sufficient criterion of reality and illusion. The supporters of this theory find no difficulty in admitting the reality of the phenomena of the internal sense in time; nay, they go the length of maintaining that this internal experience is of itself a sufficient proof of the real existence of its object as a thing in itself." His point in the paragraph from which you are excerpting in your second paragraph is to show that he can make a distinction between reality and illusion while maintaining TI. Thus the paragraph concludes: "That there may be inhabitants in the moon, although no one has ever observed them, must certainly be admitted; but this assertion means only, that we may in the possible progress of experience discover them at some future time. For that which stands in connection with a perception according to the laws of the progress of experience is real. They are therefore really existent, if they stand in empirical connection with my actual or real consciousness, although they are not in themselves real, that is, apart from the progress of experience."

    I grant that in the paragraph from which you're excerpting, there may be sentences that look like compressed arguments for TI: "Nay, the sensuous internal intuition of the mind (as the object of consciousness), the determination of which is represented by the succession of different states in time, is not the real, proper self, as it exists in itself—not the transcendental subject—but only a phenomenon, which is presented to the sensibility of this, to us, unknown being. This internal phenomenon cannot be admitted to be a self-subsisting thing; for its condition is time, and time cannot be the condition of a thing in itself. But the empirical truth of phenomena in space and time is guaranteed beyond the possibility of doubt, and sufficiently distinguished from the illusion of dreams or fancy—although both have a proper and thorough connection in an experience according to empirical laws. The objects of experience then are not things in themselves, but are given only in experience, and have no existence apart from and independently of experience." (Between the part you quote and the end of the paragraph above.) But I don't see this as really providing the argument; rather I see it as reminding the reader of conclusions and consequences of those conclusions that were argued for at length in the Aesthetic. I am not in a position to offer a detailed interpretation of the argument of the Aesthetic here, but I take it a crucial premise has to do with the possibility of synthetic a priori knowledge and the need for TI in any account of how this is possible. And that seems to be playing no role in the argument you attribute to Kant.

    (Michael Kremer)

  6. Thanks, Michael.

    I do think Kant is giving here at least a condensed argument for (TI), but one that's based on premises he takes himself to have established earlier in TA.

    On the other hand, my understanding of TA is that, given the premise "space is the form of external intuition", then the (synthetic) a priority of our knowledge of space surely follows as well, by reflecting on our intuition. So, I take it that "space is the form of external intuition" is part of what justifies Kant's account of our synthetic a priori geometric knowledge, rather than a consequence of it. Similarly, Kant's "time is the form of our inner intuition" is what justifies our synthetic a priori knowledge of arithmetic (a view taken up by Brouwer).

    So, I'd taken Kant to be taking the crucial premises,

    (S) space is the form of external intuition
    (T) time is the form of inner intuition

    as premises in his justification of our synthetic a priori knowledge of both geometry and arithmetic, rather than consequences.

    But thanks again - I'll think about it.