 Early Greek atomism is generally taken to have been formulated in response to the Eleatic claim that ‘what is’ must be one and unchanging, because any assertion of differentiation or change within ‘what is’ involves the assertion of ‘what is not,’ an unintelligible concept. While Parmenides' argument is difficult to interpret, he was understood in antiquity to have forced philosophers after him to explain how change is possible without supposing that something comes from ‘what is not,’ i.e. nothing. Aristotle tells us that Leucippus tried to formulate a theory that is consistent with the evidence of the senses that change and motion and a multiplicity of things exists in the world (DK 67A7). In the atomist system, change only occurs at the level of appearances: the real constituents of being persist unchanged, merely rearranging themselves into new combinations that form the world of appearance. Like Parmenidean Being, the atoms cannot change or disintegrate into ‘what is not’ and each is a solid unit; nonetheless, the combinations of atoms that form the world of appearance continually alter. Aristotle cites an analogy to the letters of the alphabet, which can produce a multitude of different words from a few elements in combinations; the differences all stem from the shape (schêma) of the letters, as A differs from N; by their arrangement (taxis), as AN differs from NA; and by their positional orientation (thesis), as N differs from Z (DK 67A6).