Last Word On Idealism vs. Realism.
(a selection from “Hegel’s Dialectic and the Motion of Motion” By James Donaldson, some edits added)
The origin of [Hegel's] dialectic is really quite simple. It is merely a matter of following out the particular theory of abstraction which results when we posit that the thing in itself, or primary substance is unknowable. Its difference from Aristotle on this point is profound in many ways.
First, Aristotle does not deny our knowledge of things, and hence the knowledge we have of them, though partial and incomplete in itself, can be used to signify the whole as existing in reality. In this manner, for Aristotle, the enunciation depends for its unity not on the abstract, formal unity of the concept, but on the real underlying unity of the thing in reality. This is why Aristotle distinguishes between the modus rei, and the modus rei ut cognita (the thing known from the thing inasmuch as it is known) . The thing as existing in reality is composed of integral parts, but the parts of the thing as existing in the mind are concepts which are knowledge of the whole thing, although separately taken they only give a partial knowledge of the thing explicitly. Thus the whole as known through one partial concept can be combined with the whole known through another partial concept.
For Hegel, however, the correspondence between our partial knowledge and the whole thing in reality is denied, and our concepts are all taken as integral parts. And so for Hegel the the distinction between the thing in reality and as known falls down, and the mode of the thing existing in reality and in the mind are equated. The thing in reality is exactly as it is known. Hence, mental composition, like material composition, is of integral parts joined by the copula “is”. Of course, a concrete enunciation involving a difference between subject and predicate immediately involves a most resounding contradiction (ed. note: one part can never be said of another, e.g. “a hand is a foot”)…
Hence Hegel holds that there is a contradiction involved in in every sentence in which there is the least difference between the subject and the predicate, and he pushes his his point to say that even the enunciation of the sentence “A is A” involves a contradiction, because “A” as a subject is different from “A” as a predicate.