Next we must state what the difference is between coming-to-be and alteration-for we maintain that these changes are distinct from one another.
Since, then, we must distinguish (a) the substratum, and (b) the property whose nature it is to be predicated of the substratum; and since change of each of these occurs; there is alteration when the substratum is perceptible and persists, but changes in its own properties, the properties in question being opposed to one another either as contraries or as intermediates. The body, e.g. although persisting as the same body, is now healthy and now ill; and the bronze is now spherical and at another time angular, and yet remains the same bronze. But when nothing perceptible persists in its identity as a substratum, and the thing changes as a whole (when e.g. the seed as a whole is converted into blood, or water into air, or air as a whole into water), such an occurrence is no longer alteration. It is a coming-to-be of one substance and a passing-away of the other-especially if the change proceeds from an imperceptible something to something perceptible (either to touch or to all the senses), as when water comes-to-be out of, or passes-away into, air: for air is pretty well imperceptible. If, however, in such cases, any property (being one of a pair of contraries) persists, in the thing that has come-to-be, the same as it was in the thing which has passed-away-if, e.g. when water comes-to-be out of air, both are transparent or cold-the second thing, into which the first changes, must not be a property of this persistent identical something. Otherwise the change will be alteration. Suppose, e.g. that the musical man passed-away and an unmusical man came-tobe, and that the man persists as something identical. Now, if musicalness and unmusicalness had not been a property essentially inhering in man, these changes would have been a coming-to-be of unmusicalness and a passing-away of musicalness: but in fact musicalness and unmusicalness are a property of the persistent identity, viz. man. (Hence, as regards man, these changes are modifications; though, as regards musical man and unmusical man, they are a passing-away and a coming-to-be.) Consequently such changes are alteration. When the change from contrary to contrary is in quantity, it is growth and diminution; when it is in place, it is motion; when it is in property, i.e. in quality, it is alteration: but, when nothing persists, of which the resultant is a property (or an accident in any sense of the term), it is coming-to-be, and the converse change is passing-away.
Matter, in the most proper sense of the term, is to be identified with the substratum which is receptive of coming-to-be and passing-away: but the substratum of the remaining kinds of change is also, in a certain sense, matter, because all these substrata are receptive of contrarieties of some kind. So much, then, as an answer to the questions (i) whether coming-to-be is or is not--i.e. what are the precise conditions of its occurrence and (ii) what alteration is: but we have still to treat of growth.