Foucault, near the end of The Order of Things, with brilliant prose attacked what he called the empirical-transcendental doublet in Kant, by which Foucault meant a putative subject that is supposed to be simultaneously empirical and transcendental.
Kant is often criticized for his apparent dualisms, and with some justification. Foucault’s criticism has an opposite form. It presupposes that Kant’s distinction between empirical subjectivity and the transcendental does not really hold. If it did, there would be no confusion between the two. Here is a case where Kant’s so-called dualism is really helpful.
There is a subtlety here, because there must still be some interaction among these things that need to be distinguished. The transcendental is independent of experience, without being otherworldly. It helps shape experience, without violating that independence, in a way that is difficult to conceive and articulate. This is a huge and fertile area still in need of development, although there has been interesting work along these lines.
What Foucault’s criticism legitimately applies to is a bad Kantianism that wants to imbue empirical subjects with transcendental powers, or to use the transcendental as a foundational guarantee for some alleged properties of empirical subjectivity. Wherever there is undifferentiated talk about “the” subject, this sort of thing is likely to be at work. Kant himself is not guilty of this. (See also Archaeology of Knowledge; What Is “I”?)