 In the complex treatment of Parmenides in Physics 1.2–3, Aristotle introduces Parmenides together with Melissus as representing the position – within the doxographical schema adapted from that in Gorgias's On Nature, or On What is Not that structures his own examination of earlier archê-theories – that there is a single and unchanging archê or principle (Ph. 1.2.184b15–16). Aristotle recognizes, however, that this grouping obscures very real differences between the two thinkers’ views. According to Aristotle, Melissus held that everything is a single, i.e. continuous or indivisible, and unlimited quantity (or extension). Parmenides, on Aristotle’s reconstruction, recognized only a use of “being” indicating what something is in respect of its substance or essence; he accordingly supposed that everything that is is substance, and he supposed everything to be one in the sense that the account of the essence of everything is identical. Furthermore, on Aristotle’s view of Parmenides, whatever might differentiate what is cannot do so with respect to its essence but only accidentally. But no accident of what just is can belong to its essence, and since Parmenides admits only a use of “being” indicating what something is in respect of its substance or essence, no differentiating accident of what is can be said to be. Such is the thrust of Aristotle’s reconstruction of Parmenides’ reasoning at Physics 1.3.186a34-b4 and, likewise, of his summary allusion to this passage at Metaphysics 1.5.986b28–31.