On Generation and corruption   

differences signify more a 'this somewhat', is itself more

'substantial' or 'real': while a material, whose constitutive

differences signify privation, is 'not real'. (Suppose, e.g. that 'the

hot' is a positive predication, i.e. a 'form', whereas 'cold' is a

privation, and that Earth and Fire differ from one another by these

constitutive differences.)

The opinion, however, which most people are inclined to prefer, is

that the distinction depends upon the difference between 'the

perceptible' and 'the imperceptible'. Thus, when there is a change

into perceptible material, people say there is 'coming-to-be'; but

when there is a change into invisible material, they call it

'passing-away'. For they distinguish 'what is' and 'what is not' by

their perceiving and not-perceiving, just as what is knowable 'is' and

what is unknowable 'is not'-perception on their view having the

force of knowledge. Hence, just as they deem themselves to live and to

'be' in virtue of their perceiving or their capacity to perceive, so

too they deem the things to 'be' qua perceived or perceptible-and in

this they are in a sense on the track of the truth, though what they

actually say is not true.

Thus unqualified coming-to-be and passingaway turn out to be

different according to common opinion from what they are in truth. For

Wind and Air are in truth more real more a 'this somewhat' or a

'form'-than Earth. But they are less real to perception which explains

why things are commonly said to 'pass-away' without qualification when

they change into Wind and Air, and to 'come-to-be' when they change

into what is tangible, i.e. into Earth.

We have now explained why there is 'unqualified coming-to-be'

(though it is a passingaway-of-something) and 'unqualified passingaway

(though it is a coming-to-be-of-something). For this distinction of

appellation depends upon a difference in the material out of which,

and into which, the changes are effected. It depends either upon

whether the material is or is not 'substantial', or upon whether it is