The matter, then, is the absolutely formless. It is the substratum which underlies everything. It has, in itself, no character. It is absolutely featureless, indefinite, without any quality. Whatever gives a thing definiteness, character, quality, whatever makes it a this or that, is its form. Consequently, there are no differences within matter. One thing can only differ from another by having different qualities. And as matter has no qualities, it has no difference. And this in itself shows that the Aristotelian notion of matter is not the same as our notion of physical substance. For, according {279} to our modern usage, one kind of matter differs from another, as brass from iron. But this is a difference of quality, and for Aristotle all quality is part of the form. So in his view the difference of brass from iron is not a difference of matter, but a difference of form. Consequently, matter may become anything, according to the form impressed upon it. It is thus the possibility of everything, though it is actually nothing. It only becomes something by the acquisition of form. And this leads directly to a most important Aristotelian antithesis, that between potentiality and actuality. Potentiality is the same as matter, actuality as form. For matter is potentially everything. It may become everything. It is not actually anything. It is a mere potentiality, or capacity of becoming something. But whatever gives it definiteness as a this or that, whatever makes it an actual thing, is its form. Thus the actuality of a thing is simply its form. Aristotle claims, by means of the antithesis of potentiality and actuality, to have solved the ancient problem of becoming, a riddle, propounded by the Eleatics, which had never ceased to trouble Greek thinkers. How is becoming possible? For being to pass into being is not becoming, for it involves no change, and for not-being to pass into being is impossible, since something cannot come out of nothing. For Aristotle, the sharp line drawn between not-being and being does not exist. For these absolute terms he substitutes the relative terms potentiality and actuality, which shade off into each other. Potentiality in his philosophy takes the place of not-being in previous systems. It solves the riddle because it is not an absolute not-being. It is {280} not-being inasmuch as it is actually nothing, but it is being because it is potential being. Becoming, therefore, does not involve the impossible leap from nothing to something. It involves the transition from potential to actual being. All change, all motion, is thus the passage of potentiality into actuality, of matter into form.