In the third of his 1903 Lowell Lectures, Peirce has the highest praise for Kant (CP 1.522). Yet he includes Kant in his remark that ‘all modern philosophy of every sect has been nominalistic’ (CP 1.19). ‘Kant was a nominalist; although his philosophy would have been rendered compacter, more consistent, and stronger if its author had taken up realism, as he certainly would have done if he had read Scotus’. Of course Peirce himself was a realist, as he went on to explain (CP 1.20):
In a long notice of Frazer's Berkeley, in the North American Review for October, 1871, I declared for realism. I have since very carefully and thoroughly revised my philosophical opinions more than half a dozen times, and have modified them more or less on most topics; but I have never been able to think differently on that question of nominalism and realism.
According to Peirce's Berkeley review, a realist like himself will ‘deny that there is any reality which is absolutely incognizable in itself, so that it cannot be taken into the mind’ (CP 8.13). Thus he would reject the idea of a ‘Kantian thing-in-itself’ (CP 6.108, 1892). Is this where Peirce parted company with Kant? He wrote in 1905 that the ‘Kantist has only to abjure from the bottom of his heart the proposition that a thing-in-itself can, however indirectly, be conceived; and then correct the details of Kant's doctrine accordingly, and he will find himself to have become a Critical Common-sensist’ (CP 5.452).
Yet Peirce appears to take a different perspective on Kant in the 1871 Berkeley review itself (CP 8.15):
what Kant called his Copernican step was precisely the passage from the nominalistic to the realistic view of reality. It was the essence of his philosophy to regard the real object as determined by the mind. That was nothing else than to consider every conception and intuition which enters necessarily into the experience of an object, and which is not transitory and accidental, as having objective validity. In short, it was to regard the reality as the normal product of mental action, and not as the incognizable cause of it.
Is this compatible with Peirce's later remarks about Kant's nominalism and the ‘Kantian thing-in-itself’? We might try to account for the differences by guessing that he changed his mind about Kant after 1871. But in a later lecture from the same 1903 Lowell series quoted above, Peirce returned to the subject of Kant in a fashion very similar to his 1871 remarks. He did so in a passage which he described in advance as ‘so brief that only the most thorough student of philosophy could fully grasp the meaning of it at the single hearing.’ Since it is so brief, i will quote it here in full:
The first thing to be taken into consideration is the general upshot of Kant's Critic of the Pure Reason. The first step of Kant's thought – the first moment of it, if you like that phraseology – is to recognize that all our knowledge is, and forever must be, relative to human experience and to the nature of the human mind. That conception being well digested, the second moment of the reasoning becomes evident, namely, that as soon as it has been shown concerning any conception that it is essentially involved in the very forms of logic or other forms of knowing, from that moment there can no longer be any rational hesitation about fully accepting that conception as valid for the universe of our possible experience. To repeat an example I have given before, you look at an object and say ‘That is red.’ I ask you how you prove that. You tell me you see it. Yes, you see something; but you do not see that it is red; because that it is red is a proposition; and you do not see a proposition. What you see is an image and has no resemblance to a proposition, and there is no logic in saying that your proposition is proved by the image. For a proposition can only be logically based on a premiss and a premiss is a proposition. To this you very properly reply, with Kant's aid, that my objections allege what is perfectly true, but that instead of showing that you have no right to say the thing is red they conclusively prove that you are logically justified in doing so. At this point, the idealist appears before the tribunal of your reason with the suggestion that since these metaphysical conceptions, that repose upon their being involved in the forms of logic, are only valid for experience and since all our knowledge is relative to the human mind, they are not valid for things as they objectively are; and since the conception of existence is preeminently a conception of that description, it is a mere fairy tale to say that outward objects exist, the only objects of possible experience being our own ideas. Hereupon comes the third moment of Kant's thought, which was only made prominent in the second edition, not, as Kant truly says, that it was not already in the book, but that it was an idea in which Kant's mind was so completely immersed that he failed to see the necessity of making an explicit statement of it, until Fichte misinterpreted him. It is really a most luminous and central element of Kant's thought. I may say that it is the very sun round which all the rest revolves. This third moment consists in the flat denial that the metaphysical conceptions do not apply to things in themselves. Kant never said that. What he said is that these conceptions do not apply beyond the limits of possible experience. But we have direct experience of things in themselves. Nothing can be more completely false than that we can experience only our own ideas. That is indeed without exaggeration the very epitome of all falsity. Our knowledge of things in themselves is entirely relative, it is true; but all experience and all knowledge is knowledge of that which is, independently of being represented. Even lies invariably contain this much truth, that they represent themselves to be referring to something whose mode of being is independent of its being represented. This is true even if the proposition relates to an object of representation as such. At the same time, no proposition can relate, or even thoroughly pretend to relate, to any object otherwise than as that object is represented. These things are utterly unintelligible as long as your thoughts are mere dreams. But as soon as you take into account that Secondness that jabs you perpetually in the ribs, you become awake to their truth. Duns Scotus and Kant are the great assertors of this doctrine, for which Thomas Reid deserves some credit too. But Kant failed to work out all the consequences of this third moment of thought and considerable retractions are called for, accordingly, from some of the positions of his Transcendental Dialectic. Nor in other respects must it be supposed that I assent to everything either in Scotus or in Kant. We all commit our blunders.(CP 6.95)
Perhaps it is my blunder to suggest that the various remarks about Kant quoted above are somehow incompatible. But i confess that i don't quite see how they can all be included in a single consistent view. On the other hand, i can't take very seriously the idea that Peirce changed his mind about Kant's nominalism and then changed it back again. Probably i am missing some nuances of Peirce's logic here, or just don't understand how his realism relates to Kant's nominalism. If so, maybe some intrepid blog reader (nobody else would have got this far!) can straighten it all out for me (and possibly for others too).
Even if that doesn't happen, i think the long quote just above (CP 6.95) is worth several readings, as an elucidation of Peirce's realism. So maybe that's enough to justify this post.