The aim of this paper is to clarify what the word hekaston signifies in Aristotle's theory of essence, as it is developed in his Metaphysics Z4-6 and 10-12. In Z4, Aristotle refers to "you, " an individual substance, as an example of hekaston, in order to explain the essence of hekaston. However, if we interpret the word hekaston to signify individual substance, it becomes difficult to understand the claim in Z6 that hekaston is to be identified with its essence. For the individual substance has essence as its essential aspect and accidents as its non-essential aspects. The alternative interpretation is that the word hekaston signifies form. But this interpretation is not appropriate to the context of Z4-6, where Aristotle does not take hylomorphism into consideration. So I suggest a third interpretation, according to which the word hekaston signifies an individual substance that is tentatively treated as that which has no accident. The advantage of this interpretation is that the word signifies such an individual substance throughout Z4-6. Furthermore, this interpretation helps us to understand the difference between the viewpoint of the first half of the theory of essence and that of the latter half.

In this paper, I shall consider the meaning of the idea that matter is potentiality and form is actuality by dealing with the problem of the unity of matter and form in Aristotle's Metaphysics H 6 According to the orthodox interpretation of H 6, Aristotle attempts to solve the problem of the unity of the definition signified by genus and differentia It is true that Aristotle refers to the definition of 'man' as 'two-footed animal', but the reference to H3 at the beginning of H 6 shows us that he is concerned with the problem of the unity of the definition combining matter and form In fact, Aristotle tentatively defines 'cloak' as 'round bronze' in order to show the problem of the unity of a definition So it is the problem of the unity of the definition composed of matter and form that should be solved by the concepts of potentiality and actuality in H 6 But Aristotle turns to the problem of the unity of matter and form, not of the definition composed of them For in his view, to give an account of the unity of matter and form is to give an account of the unity of the definition composed of them In connection with this, how should we understand the unity of the composite of matter and form ? Is it the same as the existence of a particular substance? To answer these questions, I turn to the argument of Z 17 before I examine Aristotle's solution of the problem of the unity of matter and form Aristotle's concern in Z 17 is to seek the cause by which matter is something For example, these bricks and stones are a house by the essence or the form of the house Moreover, Aristotle argues that the essence or the form is the primary cause of the existence of a particular substance by pointing out that the status of the cause is different from that of the material elements In other words, the essence or the form is the cause by which the composite of matter and form exists So what is sought in Z 17 is the cause of the existence of a composite But Aristotle treats the problem of the unity of matter and form in H 6 and he says that the unity of matter and form has no cause except the efficient cause Then the unity of matter and form should be distinguished from the existence of a composite Aristotle solves the problem by applying the concepts of potentiality and actuality to matter and form I examine how he does that by interpreting the controversial passage in which he gives us a concrete example And I find that we fall into difficulty when taking a sphere to be the composite of e g bronze and sphericality Then I explain the significance of 'what is potentially sphere being actually sphere'