Aristotle, Metaphysics

[989a] [1] and this will be that body which is rarest and composed of the finest particles.Hence all who posit Fire as first principle will be in the closest agreement with this theory. However, even among the other thinkers everyone agrees that the primary corporeal element is of this kind. At any rate none of the Monists thought earth likely to be an element—obviously on account of the size of its particles—but each of the other three has had an advocate; for some name fire as the primary element, others water, and others air.¹ And yet why do they not suggest earth too, as common opinion does? for people say "Everything is earth."And Hesiod too says² that earth was generated first of corporeal things—so ancient and popular is the conception found to be. Thus according to this theory anyone who suggests any of these bodies other than fire, or who assumes something "denser than air but rarer than water,"³ will be wrong.On the other hand if what is posterior in generation is prior in nature, and that which is developed and combined is posterior in generation, then the reverse will be the case; water will be prior to air, and earth to water. So much for those who posit one cause such as we have described.

[20] The same will apply too if anyone posits more than one, as e.g. Empedocles says that matter consists of four bodies;objections must occur in his case also, some the same as before, and some peculiar to him. First, we can see things being generated from each other in a way which shows that fire and earth do not persist as the same corporeal entity. (This subject has been treated in my works on Natural Science.⁴) Again with regard to the cause of motion in things, whether one or two should be assumed, it must not be thought that his account is entirely correct or even reasonable.⁵And in general those who hold such views as these must of necessity do away with qualitative alteration; for on such a theory cold will not come from hot nor hot from cold, because to effect this there must be something which actually takes on these contrary qualities: some single element which becomes both fire and water—which Empedocles denies.

If one were to infer that Anaxagoras recognized two⁶ elements, the inference would accord closely with a view which, although he did not articulate it himself, he must have accepted as developed by others.To say that originally everything was a mixture is absurd for various reasons,