ABSTRACT: The discussion of Heidegger's destructive retrieve of Aristotle has been intensified in recent years by the publication of Heidegger's courses in the years surrounding his magnum opus. Heidegger's explicit commentary on Aristotle in these courses permits one to read Being and Time with Aristotle's Nicomachean Ethics and Metaphysics. My paper analyzes a network of differences between the two thinkers, focusing on the relationship between theory and praxis. From Aristotle to Heidegger, there is: (1) a shift from the priority of actuality to the priority of possibility. This shift, I argue, is itself the metaphysical ground of: (2) a shift from the priority of theory to the priority of praxis. This shift is seen most clearly in the way in which (3) Heidegger's notion of Theorie is a modification of his poesis. The temporal ground of the reversal is seen in (4) Heidegger's notion of transcendence towards the world, and not towards an eternal being. I. From Aristotle's Actuality to Heidegger's Possibility For Heidegger, possibility precedes actuality: though human beings have a factical structure, the way that we interpret the world is on the basis of possibility. For Aristotle, however, actuality is prior to potentiality (Meta: 1049b 4ff). Now Aristotle's notion of physical potentiality and what Heidegger calls possibility are not identical. But nor is Aristotle's notion of potentiality limited to the coming-to-be of form in sensible things. In Metaphysics Theta, Aristotle distinguishes between rational potencies [dunmeis logoi] and irrational potencies [dunmeis logoi]. Irrational potencies are those that admit of only one result, thus something hot must produce heat; this is the physical and metaphysical notion of potency, that everything tends, by nature, towards fulfillment of its potency. In the physical world, sensible ousa are finite as particulars, but the universal character of ousa, the universal form, is eternal. The physical cosmos is the eternal cycle of movement from potentiality to actuality in sensible things. In book Lambda, it is shown that this eternal movement is grounded formally and finally by a first mover, an eternal being, who is fully actual. Thus actuality precedes potentiality in the physical cosmos. The notion of rational potency is perhaps closer to what Heidegger intends by possibility. Rational potencies, such as the tchnai (the poietic sciences), admit of contrary results: the science of medicine can produce sickness or health. The actualization of rational potencies is determined by desire (particularly in the case of animals) or by rational choice [rexin proaresin], though it depends on whether the desire or choice is directed towards that for which we have a given capacity (Cf: Meta: 1046a 36-b 9; 1047b 31-1048a 15). But desire and choice are also directed towards some end. Choice is the efficient explanation of action; and the final explanation of choice is desire and reason, themselves directed to some end (NE:1139a 6).