 Socrates argues in the Protagoras for the unity of virtues, a thesis that has been variously interpreted and may be supported by other dialogues in this group. Vlastos famously explained the thesis as bi-conditional: that whoever has one virtue has them all. Others (Penner, Woodruff) have argued for a stronger thesis, such that the definitions of all the virtues would have a common essence. Dialogues of definition, such as the Laches, seem to presuppose that each virtue has its own definition, but, at the same time, such dialogues seem to be moving towards the view that the essence of each virtue is wisdom or knowledge. The Protagoras defines courage as the knowledge of what is and is not to be feared (360d), but the Laches rejects a similar definition on the grounds that it does not differentiate courage adequately from other virtues (199e). And the Euthyphro ends in aporia (at an impasse) because Socrates is unable to differentiate reverence adequately from justice. Efforts to define sophrosune (integrity or temperance) in the Charmides end in an impasse partly owing to the difficulty of specifying the subject of the knowledge that is the essence of this virtue. If (as seems the case) Socrates is never satisfied with a definition of a virtue, this may be because the virtues cannot be differentiated.