In Book I Chapter 8 of the Physics, Aristotle presents us with the position of previous philosophers who maintained that coming to be (motion) is impossible. The argument runs thus:
What comes to be must come to be either from what is or from what is not.
What is cannot come to be since it already is.
What is not cannot come to be because something cannot come from nothing.
Therefore coming to be is impossible.
Aristotle's solution rests within a distinction. There is a sense in which the previous philosophers are right, something cannot come from nothing. However, there are two ways of 'coming from nothing.' One way is simply and without qualification. If this is the only sense admitted then the previous philosophers are right and motion is impossible. This is not the only sense admitted, though. There is also coming to be with qualification. A man does not become pale simply, he comes to be pale after being dark. Notice that the term 'coming to be from nothing' becomes ambiguous. The dark man is not pale, and in this way paleness comes to be from nothing. The difference is that it is not nothing taken absolutely but rather as a privation.