Why should one write on Heidegger's understanding of logic? After all, Heidegger was not a logician, nor did he do philosophy of logic. Indeed, there is no justification for expecting of any great philosopher whatsoever that he should have views, and reasonably plausible views, about the nature of logic or on specific themes belonging to the domain of logic. A moral philosopher may totally bypass any concern with logic, without detriment to his thinking. As an existentialist philosopher, Heidegger could have done that, and much of his Dasein-analytic would yet have retained its value. But Heidegger was also an ontologist, and was deeply concerned, all his philosophical career, with metaphysics and with the various questions about the nature of thought and being. These concerns, to say the least, bring him to the proximity of logic as it had been understood in the tradition going back to Aristotle. And, as a matter of fact, Heidegger's own access to the problems of ontology and metaphysics has been determined by his reflections on logic. Two claims may therefore be advanced. First, it is not unreasonable, and what is more important, not unfair to Heidegger, to enquire into his understanding of logic. Secondly, his reflections on logic may help us to gain a better understanding of his overall philosophical interests than would be possible otherwise. Even if he was not a logician he was concerned with the nature of logic, and with some central problems belonging to the domain of logic. This concern begins with his doctoral work on the problem of psychologism in the theory of judgment,¹ continues in the habilitation work on the semantic categories in Duns Scotus², and reaches its maturity in the Marburg lectures of 1925–28.³