But when nothing perceptible persists in its identity as a substratum, and the thing changes as a whole (when eg the seed as a whole is converted into blood, or water into air, or air as a whole into water), such an occurrence 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.