Kant stated in Critique of Pure Reason, pg. 273:

What our understanding acquires through this concept of a noumenon, is a negative extension; that is to say, understanding is not limited through sensibility; on the contrary, it itself limits sensibility by applying the term noumena to things in themselves (things not regarded as appearances). But in so doing it at the same time sets limits to itself, recognising that it cannot know these noumena through any of the categories, and that it must therefore think them only under the title of an unknown something.

I'm not sure whether Kant meant to say that the 'thing-in-itself' limits understanding or that our concept of the 'thing-in-itself' (which he called 'noumenon') limits understanding.

C.S. Peirce said things in several places that seem to align his own view with the before-mentioned Kantian view:

The Ding an sich, however, can neither be indicated nor found. Consequently, no proposition can refer to it, and nothing true or false can be predicated of it. Therefore, all reference to it must be thrown out as meaningless surplusage (CP, 5.525).

Peirce even stated in the same collection:

His [i.e. Kant's] limitation of them [i.e. space, time, and the twelve categories] to possible experience is pragmaticism in the general sense.

What makes Peirce especially difficult to interpret in a linear way however is that he considered himself a scholastic realist:

I am myself a scholastic realist of a somewhat extreme stripe (CP, 5.470).

In what way did Peirce think the 'thing-in-itself' acted as limit to human understanding? How much of this did he borrow from Kant and if he diverged from Kant, in what way?

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    Nitpick: We can know things only as they appear and cannot know anything about the thing-in-itself, not even whether it appears exactly as it "really" is, which is very well possible, as in the end it "is" only insofar it is as a thing appearing. One should not forget that Kant himself was more clear later on (opus posthumum) regarding things essentially always appearing and only in appearance being a thing proper.
    – Philip Klöcking
    Nov 4, 2017 at 20:30
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    @PhilipKlöcking. The idea of appearing "as it really is" is incomprehensible to me because it refers to it as it is independent of any appearance. I have no means of attaching any meaning to that, especially since it appears to be contradictory — to appear as it does not appear at the same time and in the same sense. I might think of knowing the thing-in-itself as knowing it as God knows it, but that is completely beyond my conceptual framework.
    – user3017
    Nov 4, 2017 at 20:50
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    @PédeLeão: To be more exact - Kant himself was quite ambiguous at times, but many scholars looking at all of the opus (not just CPR), not the least of them Guyer, agree that the thing-in-itself has no ontological status whatsoever (i.e. is no thing, or real at all!), but only, as a concept, tries to point out that speaking of things, we should be clear that we speak of things appearing. It fulfills a logical function and nothing more should be attached to it. Reality, world, etc. are all concepts that only include objects of experience (or make them possible). For Kant, that is.
    – Philip Klöcking
    Nov 4, 2017 at 21:06
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    See Kant-Lexikon (Willaschek et al), p. 1688 (I translate/paraphrase here): Noumenon is only permissible in a negative understanding (CPR B307ff.;A252;A255f./B311f.;A286f./B342f.), which means in abstraction from the "manner of our intuition of it" (CPR, B307). Since you cannot refer to real objects without intuition, noumena in this understanding are mere "things of thought" [Gedankendinge] (CPR A290ff./B347f.; see also 22:31f.;22:36; 22:414), i.e. internal representations of non-contradictory concepts, and can only be used problematically, (1/2)
    – Philip Klöcking
    Nov 4, 2017 at 21:41
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    i.e. as a concept that "that contains no contradiction but [whose] [...] objective reality can in no way be cognized [dessen objective Realität aber auf keine Weise erkannt werden kann]" (CPR A254/B310; see A256/B311;A286/B343) (2/2)
    – Philip Klöcking
    Nov 4, 2017 at 21:42

1 Answer 1


Interpretation of Peirce's realism which grew out of combining Kantian epistemology with scholastic ontology of Duns Scotus (Peirce calls himself "a scholastic realist of a somewhat extreme stripe") is indeed difficult. What makes it even more difficult is that Peirce went through several major reworkings of his "architectonic" without clearly indicating what was kept and what was revised, so it is easy to find statements in his works that appear to be contradictory. It is fair to say, however, that he flatly denies the intelligibility of "things in themselves", as did many others starting with Fichte, "there can be no concept of absolutely incognizable since nothing of this sort occurs in experience... Hence a term can have no such meaning".

But Peirce also rejected Kant's identification of things in themselves with noumena. Pihlström in Peircean Scholastic Realism and Transcendental Arguments argues that Peirce can be understood as naturalizing and pragmatizing Kantian transcendental arguments, and his noumena are projections of "pure reason" downgraded to fallible human knowledge. Due to Kantian scruples Peirce can not adopt a metaphysical realism with already conceptualized world "grasped" by the mind (he rejects what is now called the myth of the Given). On the other hand, he wants a robust notion of truth answerable to independent and demystified reality. The result is a dual picture of reality, first as non-conceptual and encountered through action and reaction (pragmatism), and second as conceptualized through such encounters but only "at the end of inquiry". The inquiry may involve multiple human generations, and potentially even non-human beings, and may never be achieved in actuality, Peirce defends reality of possibilia. The truth is identified with this "final opinion", hence it is not a correspondent truth in any traditional sense. And it is this final opinion that hosts the noumena:

"There is a definite opinion to which the mind of man is, on the whole and in the long run tending. On many questions the final agreement is already reached, on all it will be reached if time enough is given... This final opinion, then, is independent, not indeed of thought, in general, but of all that is arbitrary and individual in thought; is quite independent of how you, or I or any number of men think. Everything, therefore, which will be thought to exist in the final opinion is real, and nothing else...

This theory of reality is instantly fatal to the idea of a thing in itself, - a thing existing independent of all relation to the mind's conception of it. Yet it would by no means forbid, but rather encourage us, to regard the appearances of sense as only signs of the realities. Only, the realities which they represent, would not be the unknowable cause of sensation, but noumena or intelligible conceptions which are the last products of the mental action which is set in motion by sensation". [CP 8.12-13, emphasis Peirce's]

Margolis in The Passing of Peirce's Realism rightly points out that in the absence of metaphysical guarantees it is unclear why the inquiry should converge on any "final opinion". Peirce himself moved from certainty about this to merely "hope" in late years. Aside from being written off as "19th century optimism", the usual defense is pragmatic, this convergence is merely a hypothesis, as the entire corpus of science is hypothetical, albeit confirmed by practice. This is not far from Kant's regulative ideas or neo-Kantian "limit concepts".

Pragmatism even allows Peirce to offer hypothetical metaphysics, which connects his remote noumena to the acting/reacting reality of everyday encounters, see Haack's "Extreme Scholastic Realism:" Its Relevance to Philosophy of Science Today. It posits actions/processes at the core of experienced reality (Secondness), objects are viewed as their conceptual derivatives, and it construes "generals", relations and laws (Thirdness), as manifested in the patterns of actions/reactions. It is they that are the fundamental reality for Peirce:"All that Hume attacked I defend, namely, law as a reality", this is why he calls his realism "of extreme stripe". This relational ontology might raise concerns familiar from dealing with Platonic froms, infinite regress on relations of relations, etc. But for Peirce only some generals are real, which ones is for empirical science to hypothesize and confirm in action, it is not a business of a priori Platonic contemplation with its pure mathematical largesse.

Pihlström suggests that one world/two perspectives (a.k.a. dual aspect) interpretation of Kant is more favorable to finding common ground with Peirce than the traditional phenomenalist interpretation that Peirce accepted. Indeed, this reading is more hospitable to realism since appearances are already real and things-in-themselves are just another aspect of them. Two key differences still remain however. First, Kant's picture is static, we have a fixed transcendental subject confronting a fixed world dressed into a straightjacket of reason's own fixed a priori categories and schemata. There is little epistemic movement in this world beyond clarification of the obscure. Kant can not therefore make the use Peirce has for noumena as happy limits of epistemic evolution, they are static empty posits.

Second, Kant's view of knowledge is absolutist, nothing short of apodictically certain passes the muster, not metaphysics, not chemistry, not psychology. On this standard the chasm between phenomena and noumena is indeed unbridgeable. But on Peirce's pragmatic view even mathematical certainty is fallible, and its difference with the empirically scientific one is of a degree, not kind. Even the "a priori" that frame phenomena are neither a priori nor immutable, they are merely high level hypotheses to be revised eventually. So what Kant cast out as less-than-knowledge aided by practical reason and reflective judgment, is now admitted in full and paves the road from phenomena to noumena. On Peirce's optimistic "hope" the two perspectives merge in the "final opinion".

  • Thanks for your well thought out reply. You briefly state that "Peirce also rejected Kant's identification of things in themselves with noumena." From the passage I used from Kant it seems as though there is wiggle room for how Kant should be interpreted in his identification of the noumenon with the thing-in-itself. One viable interpretation would seem to be that Kant thought noumena were not things-in-themselves but acts of understanding. I'd appreciate it if you could clarify your interpretation of Kant in comparison to Peirce.
    – Byday
    Nov 5, 2017 at 16:45
  • Wow. I visited the SEP on Pierce after the question was asked, and found a little bit about it in para. 25 or 26, but it left me in complete confusion. Hats off to Byday too for asking this question as an English major!! Great comments above on Kant too. I still don't fully understand this, but I do know that hours of research time have been saved by this Q&A. I have already upvoted this answer, and Bydays question.
    – Gordon
    Nov 5, 2017 at 17:01
  • @Gordon One thing an English major helps you out with is getting through confusing literature. After reading James Joyce everything else seems crystal clear. I'm in the same boat as you with most of the literature on idealism though. I'm having to mull over a lot parts in Conifold's response.
    – Byday
    Nov 5, 2017 at 17:08
  • @Byday That's right, I didn't consider Joyce! You should feel right at home here. The part of the answer that is most helpful for me is this... On the other hand, he wants a robust notion of truth answerable to independent and demystified reality. The result is a dual picture of reality, first as non-conceptual and encountered through action and reaction (pragmatism), and second as conceptualized through such encounters but only "at the end of inquiry"... very helpful and significant.
    – Gordon
    Nov 5, 2017 at 17:23
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    @Byday The Kantian one you mentioned, between metaphysical realism and transcendental idealism (Kant's use of "idealism" is non-traditional, it is closer to what is now called anti-realism). I added a passage on comparing Peirce to Allison's interpretation that you seem to favor, hopefully it would help.
    – Conifold
    Nov 6, 2017 at 19:49

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