Full entry here.

The previous point must be tempered with the fact that Peirce increasingly became a philosopher with broad and deep sympathies for both transcendental idealism and absolute idealism. His Kantian affinities are simpler and easier to understand than his Hegelian leanings. Having rejected a great deal in Kant, Peirce nevertheless shared with Charles Renouvier the view that Kant’s (quasi-)concept of the Ding an sich can play no role whatsoever in philosophy or in science other than the role that Kant ultimately assigned to it, viz. the role of a Grenzbegriff: a boundary-concept, or, perhaps a bit more accurately, a limiting concept. A supposed “reality” that is “outside” of every logical possibility of empirical or logical interaction with “it” can play no direct role in the sciences. Science can deal only with phenomena, that is to say, only with what can “appear” somehow in experience. All scientific concepts must somehow be traceable back to phenomenological roots. Thus, even when Peirce calls himself a “realist” or is called by others a “realist,” it must be kept in mind that Peirce was always a realist of the Kantian “empirical” sort and not a Kantian “transcendental realist.” His realism is similar to what Hilary Putnam has called “internal realism.” (As was said, Peirce was also a realist in quite another sense of he word: he was a realist or an anti-nominalist in the medieval sense.)

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