Institutio Oratoria

For example, a woman who is delivered of a child must have had intercourse with a man, and the reference is to the past. When there is a high wind at sea, there must be waves, and the reference is to the present. When a man has received a wound in the heart, he is bound to die, and the reference is to the future. Nor again can there be a harvest where no seed has been sown, nor can a man be at Rome when he is at Athens, nor have been wounded by a sword when he has no scar. Some have the same force when reversed:

a man who breathes is alive, and a man who is alive breathes. Some again cannot be reversed: because he who walks moves it does not follow that he who moves walks.