(Mehta1976)
The Principle of Identity does not, Heidegger claims, merely say that every A is itself the same but rather that every A is with itself the same (idem, to auto, das Selbe; Identity as distinguished from sameness). In self-sameness there lies the relation of ‘with’, hence a mediation, a union, a synthesis, unification into a unity. This is the reason why, throughout the history of Western thought, Identity appears in the character of Unity. But this Unity is by no means the monotonous vacuity of what, in itself without relation, persists unwearyingly in its indifferent oneness. But Western thought needs more than two thousand years until the relation of the same with itself, lying within Identity and already glimmering in the beginning, comes to light definitely and in its characteristic form. Only the philosophy of Speculative Idealism, prepared by Leibniz and Kant, gives accommodation, through Fichte, Schelling, and Hegel, to the intrinsically synthetic nature of Identity. After the epoch of Speculative Idealism it is no longer permissible for thought to conceive the unity of Identity as mere sameness and to ignore the mediation inherent in unity. Where this happens, Identity is conceived only abstractly. According to Heidegger, it may be added, the Principle of Identity, like the Principle of Sufficient Reason, refers primarily to the Being of what is and only consequentially to thought. “The Principle holds as a law of thought only insofar as it is a Law of Being.” See Heidegger’s essay “Grundsätze des Denkens” (in Jahrbuch für Psychologie und Psychotherapie 6 (1958), for an examination of the laws of thought in relation to the dialectical thought of Hegel and Marx.