Meaning - 'the subject can even be predicated of itself'

Source: Ch 3, Section 2, Critique of Pure Reason, by Kant, translated by Norman Kemp Smith

I should have hoped to put an end to these idle and fruitless disputations in a direct manner, by an accurate determination of the concept of existence, had I not found that the illusion which is caused by the confusion of a logical with a real predicate (that is, with a predicate which determines a thing) is almost beyond correction. Anything we please can be made to serve as a logical predicate; the subject can even be predicated of itself; for logic abstracts from all content. But a determining predicate is a predicate which is added to the concept of the subject and enlarges it. Consequently, it must not be already contained in the concept.

predicate = [with object] 1. {Grammar & Logic} state, affirm, or assert (something) about the subject of a sentence or an argument of a proposition

I recast user John Lawler's comment: Denote A a human agent, P a predicate; X the argument of P.
A predicates P of X   =   P is predicated by A of X   =    Say( A, P(X) ).

Yet I remain confused. I don't see any human agent here, How do you determine/deduce the meaning of the bolded? How can a subject predicate itself?

Footnote: This bolded phrase features as the first Example Sentence at ODO.