Those opponents of the Eleatean unmoved Unity were induced to make light of such an argument by prejudices of a perceptual character. It seems so irrefutable that each veritable "Existent" is a space-filling body, a lump of matter, large or small but in any case spacially dimensioned; so that two or more such lumps cannot be in one space. Under this hypothesis Anaxagoras, as later on Democritus, assumed that they must knock against each other; if in their motions they came by chance upon one another, that they would dispute the same space with each other, and that this struggle was the very cause of all Change. In other words: those wholly isolated, thoroughly heterogeneous and eternally unalterable substances were after all not conceived as being absolutely heterogeneous but all had in addition to a specific, wholly peculiar quality, also one absolutely homogeneous substratum: a piece of space-filling matter. In their participation in matter they all stood equal and therefore could act upon one another, _i.e.,_ knock one another. Moreover all Change did not in the least depend on the heterogeneity of those substances but on their homogeneity, as matter. At the bottom of the assumption of Anaxagoras is a logical oversight; for that which is _the_ "Existent-In-Itself" must be wholly unconditional and coherent, is therefore not allowed to assume as its cause anything,--whereas all those Anaxagorean substances have still a conditioning Something: matter, and already assume its existence; the substance "Red" for example was to Anaxagoras not just merely red in itself but also in a reserved or suppressed way a piece of matter without any qualities. Only with this matter the "Red-In-Itself" acted upon other substances, not with the "Red," but with that which is not red, not coloured, nor in any way qualitatively definite. If the "Red" had been taken strictly as "Red," as the real substance itself, therefore without that substratum, then Anaxagoras would certainly not have dared to speak of an effect of the "Red" upon other substances, perhaps even with the phrase that the "Red-In-Itself" was transmitting the impact received from the "Fleshy-In-Itself." Then it would be clear that such an "Existent" _par excellence_ could never be moved.