Kant gives a famous and crucial account of his method relative to scholastic metaphysics:
In carrying out the plan which the Critique prescribes, that is, in the future system of metaphysics, we must have recourse to the strict method of the celebrated Wolf, the greatest of all dogmatic philosophers. He was the first to point out the necessity of establishing fixed principles, of clearly defining our conceptions, and of subjecting our demonstrations to the most severe scrutiny, instead of rashly jumping at conclusions. The example which he set served to awaken that spirit of profound and thorough investigation which is not yet extinct in Germany. He would have been peculiarly well fitted to give a truly scientific character to metaphysical studies, had it occurred to him to prepare the field by a criticism of the organum, that is, of pure reason itself.
Pure reason is thus the organ of thought, that is, it is a definite structure which gives a peculiar determination to the unformed content of experience. In the same way that the human eye takes the content of experience and sees one determinate color in a sunflower leaf while a bee sees as two; or in the same way that the world that appears to go dark for us at sunset but becomes visible to nocturnal animals, so too pure reason gives a rational structure to possible experience. Again, just as the explanation of differing visual experiences of the same world traces back to different structures of the visual equipment of different animals, so too the determinations of human reason trace back to the determinate structure that reason has prior to experience.
If pure reason is what Kant says it is, his critical conclusions follow almost immediately, even apart from the more well-known arguments he gives in favor of them (like “existence is not a predicate” etc.). For Kant, experience is never of the world or of pure reason, but rather of an object whose content is from the world and whose determination is from the structure of the knowing power. If this is so, then we can only know that the world and the self exists, but never what they are. Crucially, we can never know the world as caused or uncaused, or the self as purely material or partially spiritual. All the great questions of metaphysics, along with the objections of the skeptics, are ruled out with one masterstroke. Reason no longer has anything to say about theism or atheism, materialism or idealism, nominalism or its opposites, or the corruptibility or immortality of the soul.
In terms of a hypothetical, Kant’s argument is that if pure reason is a structure that pre-exists thought and makes it possible, then we cannot know what the self is, whether the world is caused, or whether God exists or not. Kant clearly affirms the antecedent. Aristotle emphatically denies the consequent by saying everything [i.e. being] is an object of thought, and then draws the conclusion:
Since everything is a possible object of thought , mind in order, as Anaxagoras says, to dominate, that is, to know, must be pure from all admixture; for the co-presence of what is alien to its nature is a hindrance and a block: it follows that it too, like the sensitive part, can have no nature of its own, other than that of having a certain capacity. Thus that in the soul which is called mind (by mind I mean that whereby the soul thinks and judges) is, before it thinks, not actually any real thing.
De anima, III. 4
Aristotle’s comparison of intellect to sense is fine but a contrast would have made his point clearer. It’s analytic that every cognitive power lacks the form it receives before it receives it, but since a sense power only detects a limited range of forms there is no reason why it can’t have some definite formal structure before it senses. But if a sense could detect any form then it could not have any of them – but this, says Aristotle, is exactly what reason is. It is nothing before it thinks.
Another way to make the same point would be to say that there is nothing of mind that is not mind. In the same way that the mind is “pure reason” so too the ear is “pure hearing”, but the ear is a collection of things that are not ears: cochlea, auditory nerves, semicircular canals, etc. The ear is a bunch of stuff, and it’s the nature of stuff that its different from the thing it makes. But pure reason is only reason and nothing more – there is no structure or parts to it that are other than itself. It’s precisely this simplicity that is the distinctive mark of its spirituality and so also of its higher mode of existence. To exist is, again analytically, to be what one is, but there is nothing of mind that is not mind, while everything of an eye is not an eye.
The dispute between Aristotle and Kant is this: is pure reason actual a priori? Kant’s whole critical philosophy depends on saying yes; Aristotle’s whole theory of mind on saying no. This much is clear though, if mind knows either being or what it is, then it must be simple, spiritual, and with nothing of itself that is not itself. To say this, moreover, is the same as to say that it is nothing actual before it thinks.