Kitap 4
9
It is impossible for the same attribute at once to belong and not to belong to the same thing and in the same relation; and we must add any further qualifications that may be necessary to meet logical objections. This is the most certain of all principles, since it possesses the required definition;
10
for it is impossible for anyone to suppose that the same thing is and is not, as some imagine that Heraclitus saysFor examples of Heraclitus’s paradoxes cf. Heraclitus Fr. 36, 57, 59 (Bywater); and for their meaning see Burnet, E.G.P. 80.—for what a man says does not necessarily represent what he believes.
11
And if it is impossible for contrary attributes to belong at the same time to the same subject (the usual qualifications must be added to this premiss also), and an opinion which contradicts another is contrary to it, then clearly it is impossible for the same man to suppose at the same time that the same thing is and is not; for the man who made this error would entertain two contrary opinions at the same time.
12
Hence all men who are demonstrating anything refer back to this as an ultimate belief; for it is by nature the starting-point of all the other axioms as well.
