11. KINDS OF CHANGE.—Following Aristotle,(80) we may recognize a broad and clear distinction between four great classes of change (μεταβολή, _mutatio_) in the phenomena of our sense experience: local change (κίνησις κατὰ τόπον, φορά, _latio_); quantitative change (κατὰ τὸ πόσον, ἀύζησις ἤ φθίσις, _augmentatio vel diminutio_); qualitative change (κατὰ τὸ ποίον, ἀλλοίωσις, _alteratio_); and substantial change (κατ᾽ οὐσίαν, γένεσις ἤ φθορά). The three former are accidental, _i.e._ do not reach or affect the essence or substance of the thing that is changed; the fourth is substantial, a change of essence. Substantial change is regarded as taking place instantaneously, as soon as the condition brought about by the accidental changes leading up to it becomes naturally incompatible with the essence or nature of the subject. The accidental changes, on the other hand, are regarded as taking place gradually, as realizing and involving a succession of states or conditions in the subject. These changes, especially when they take place in corporeal things, are properly described as movement or motion (_motus_, _motio_). By movement or motion in the strict sense we therefore mean any change which takes place gradually or successively in a corporeal thing. It is only in a wider and improper sense that these terms are sometimes applied to activity of whatsoever kind, even of spiritual beings. In this sense we speak of thoughts, volitions, etc., as movements of the soul, _motus animae_; or of God as the Prime Mover ever in motion, the _Primum Movens semper in motu_.