 The second reason has to do with the homogeneity requirement of pure substances. Aristotle tells us that “if combination has taken place, the compound must be uniform—any part of such a compound is the same as the whole, just as any part of water is water” (DG I.10, 328a10f.). Since the elements are defined in terms of the extremes of warmth and humidity, what has intermediate degrees of these qualities is not an element. Being homogeneous, every part of a compound has the same intermediate degrees of these qualities. Thus, there are no parts with extremal qualities, and hence no elements actually present. His theory of the appearance of new substances therefore implies that the elements are not actually present in compounds.