In a much-cited essay, Barry Stroud (1968) argues that, to any claim that the truth of some proposition is a necessary condition of some fact about our mental life, the skeptic can always reply that it would be enough for it merely to appear to be true, or for us merely to believe that it is true. Transcendental arguments, he claims, at best demonstrate how things must appear, or what we must believe, rather than how things must be. Anti-skeptical transcendental arguments of familiar sorts are thus left with a gap to fill. Stroud’s contention—which is now widely accepted—is that such arguments, when aimed at refuting epistemic skepticism, can only close that gap by adverting either to a sort of verificationism or to idealism. In the case of Strawson’s arguments above, even supposing that we must be in possession of some criteria for applying concepts of other minds and/or an objective world, this fact only has anti-skeptical consequences if we also accept that there is no meaningful concept-application without experiential criteria sufficient for knowing whether the concept is instantiated. As Stroud points out, such a principle is implausible. Further, if we accepted such a principle, other aspects of transcendental arguments would be superfluous. All we would have to show is that we meaningfully employ external-world concepts; it would be impossible for any form of skepticism to be meaningful or intelligible.
As Stroud goes on to point out, another way of closing the gap between it being necessary that things appear a certain way and things being that way, would be to embrace an idealism that reduces how things are to how things appear, or must appear, to us. Kant did not rely on any verificationist principle in making the case against skepticism, but according to many scholars his “transcendental idealism” made possible the jump from how things must be experienced by us to how things must be by reducing objects of experience to mere mental representations. But such idealism is unacceptable to most: embracing idealism to answer the epistemic skeptic results in a Pyrrhic victory at best.
Despite Stroud’s blanket assertion, it should be noted that the verification/idealism objection only applies on a case-by-case basis. Some arguments that take the form of transcendental arguments may have other deficiencies, but do not rely on either verificationism or idealism. A few scholars have observed that Descartes’s “Cogito, ergo sum” argument can be re-conceived as a transcendental argument:
(1) I think.
(2) In order to think “I think,” it is necessary to exist.
(3) Hence, I exist.
This argument meets the criteria for a transcendental argument: it takes a fact about one’s mental life as a premise, adds that some extra-mental fact is a necessary condition of the truth of that premise, and concludes that the extra-mental fact holds. This argument would turn on the claim that the statement, “I do not exist
” (or better, the proposition that no one exists) is performatively self-defeating in the sense that the fact of its performance counts as conclusive evidence against its truth. That is what connects the mental fact (I am thinking about whether I exist) to the relevant extra-mental fact (I exist). Regardless of how this argument might fail in some other respect, it presupposes neither verificationism nor idealism in closing the gap between the internal and the external.