"True, I have notions, that is to say, determinate notions; but the I is the pure notion itself which, as notion, has come into existence. " (Hegel)
"Through this I or he or it (the thing) which thinks, nothing further is represented than a transcendental subject of the thoughts = X. It is known only through the thoughts which are its predicates, and of it, apart from them, we cannot have any concept whatsoever." (Kant)
You will remember that in one of the early exchanges between Iago and Roderigo in Othello Iago makes the cryptic remark "I am not what I am". It is not quite "I am not what I seem" and is the converse of the more usual (to us) radiant tautology "I am what I am". What to make of this? Typically, to say 'what you are' involves attaching a predicate to a subject: 'I am a Jew'; 'I am a writer'; 'I am a King.' The I is that about which predicates are asserted. That, for Kant, is the definition of it. But Kant's point is that none of these predicates exhausts the 'I' to which they are attached. There is always a leftover. The 'I' is that which is never fully realised in those predicates, so that these last constitutively fail to hit the mark, and this mark which they fail to hit, this mark delineated in its negativity by this failure, is the I.
If we take into account that statements of identity take the subject predicate form 'I am Y' but that the predicate never accounts for, never counts as one the 'I', then the I is not the predicate. The I is not 'what it is'. 'I am not what I am'. Iago lurks behind his predicates - 'honest Iago' , his motives even ('I am envious') - he is this pure negation, this secret naught, this silence that will never speak word. Here it is that we glimpse the modern 'I", which to the Elizabethan age is daemonic. A glint of pure negation seen in the eye of the public self.