There is no difficulty in understanding this.
None whatever.
But the question is not quite so easy when we proceed to ask whether these principles are three or one; whether, that is to say, we learn with one part of our nature, are angry with another, and with a third part desire the satisfaction of our natural appetites; or whether the whole soul comes into play in each sort of action—to determine that is the difficulty.
Yes, he said; there lies the difficulty.
Then let us now try and determine whether they are the same or different.
How can we? he asked.
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 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.