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