How can we? he asked.
The criterion of truth: Nothing can be and not be at the same time in the same relation. I replied as follows: The same thing clearly cannot act or be acted upon in the same part or in relation to the same thing at the same time, in contrary ways; and therefore whenever this contradiction occurs in things apparently the same, we know that they are really not the same, but [C]different.
Good.
For example, I said, can the same thing be at rest and in motion at the same time in the same part?
Impossible.
Still, I said, let us have a more precise statement of terms, lest we should hereafter fall out by the way. Imagine the case of a man who is standing and also moving his hands and his head, and suppose a person to say that one and the same person is in motion and at rest at the same moment128—to such a mode of speech we should object, and should [D]rather say that one part of him is in motion while another is at rest.
Very true.
Anticipation of objections to this ‘law of thought.’ And suppose the objector to refine still further, and to draw the nice distinction that not only parts of tops, but whole tops, when they spin round with their pegs fixed on the spot, are at rest and in motion at the same time (and he may say the same of anything which revolves in the same spot), his objection would not be admitted by us, because [E]in such cases things are not at rest and in motion in the same parts of themselves; we should rather say that they have both an axis and a circumference, and that the axis stands still, for there is no deviation from the perpendicular; and that the circumference goes round. But if, while revolving, the axis inclines either to the right or left, forwards or backwards, then in no point of view can they be at rest.
That is the correct mode of describing them, he replied.
Then none of these objections will confuse us, or incline us to believe that the same thing at the same time, in the [437]same part or in relation to the same thing, can act or be acted upon in contrary ways.