As usual, Wallace Stevens asks an intelligent and thoughtful question that calls for a lengthy reply. Here is what he says:
"I watched lecture three on the weekend and continue to enjoy the series. I have a comment and a question that stems from the comment. You discuss Mannheim’s idea that an ideologically-based weltanschauung can affect our perceptions of time itself, and you make the point that his idea is deeply disturbing for a Kantian. But in listening to your discussion of Mannheim’s four examples of this phenomenon, it struck me that what he is really talking about is our ideologically blinkered perceptions, or interpretations, of human history--human time and its meaning--and not time in the physical sense, as it exists independent of human events. I think that people living under, or within, all four world views would agree, for example, on notions like “before” and “after,” and the closely-related ideas of cause and effect and the irreversible, “one way” nature of time. But, and now we come to my question, was it not time in this more fundamental, physical sense that Kant had in mind when he talked about our capacity for a priori understanding? By the way, this is not a “gotcha” question--I have only the most limited understanding of these matters. But I didn’t think that Kant had a theory of human history. I thought that was more Hegel’s department."
The short answer is, "Yes, quite right," but there is a great deal more to be said. If you go back and read the eighteenth century English philosophers, you find a distinction, often repeated, between what they call Natural Philosophy, by which they mean what we would call the natural sciences, and Moral Philosophy, which encompasses not merely ethics, but History, Psychology, Political Theory, and the like. The distinction corresponds to the German distinction between naturwissenschaften and geisteswissenschaften. This distinction goes all the way back to Plato, who deploys it in the famous passage from the Phaedo in which Socrates distinguishes between the physical explanation of how he comes to be in prison awaiting his execution -- an explanation that would reference his bones and ligaments and such -- and the moral explanation of why he has chosen to accept his punishment rather the take opportunity to escape [offered to him, you will recall, by his middle-aged disciple Crito in the Dialogue of the same name.]
The discovery -- and it really was a discovery -- of a third realm, the social, between the physical and the individual, is entirely an accomplishments of the nineteenth century, at least so far as European thought is concerned, and simply cannot be found in the writings of even the greatest seventeenth and eighteenth century thinkers, such as Hume and Kant. Oh, there were histories, like those of Herodotus and Thucydides, or of Hume and Gibbon, but they were filled with the doings of prominent individuals [or representative characters of the lower orders -- see Erich Auerbach's wonderful discussions of these in Mimesis.] The social as an independent explanatory category did not exist.
Sad as I am to have to acknowledge it, it was indeed Hegel among others who introduced this category into western philosophy. It is worth taking a look at the opening and closing chapters of Ḗmile Durkheim's great work Suicide in which he, in effect, creates the discipline of Sociology be arguing that there are phenomena that can only be explained by positing an independent realm [the "Collective Unconscious" in Durkheim's view] positioned between the physical and the psychological and irreducible to either.
So Wallace Stevens is quite right -- it simply never occurred to Kant that there is a realm of phenomena whose explanation might require reference to categories that are mind-dependent but not universal. Mannheim knows all of this, of course. It is what has him so upset.