The Republic also puts forward a new theory of soul, which involves the claim that the embodied human soul has (at least) three parts or aspects, namely reason, spirit and appetite. The argument for this claim is presented in Book 4, and proceeds in roughly the following way. Socrates begins by enunciating a principle to the effect that opposite actions, affections and states cannot be assigned to one thing in respect of the same part of it, in relation to the same object and at the same time. It is then agreed that desiring and being averse are opposites, and hence that desiring to do something and being averse to doing that same thing are opposites in relation to the same object. But it does frequently happen, Socrates points out and Glaucon agrees, that the soul desires to do something and at the same time is averse to doing that same thing. This happens, for instance, when a person is thirsty and on that basis wants to drink, but at the same time wishes not to drink, on the basis of some calculation or deliberation, and in fact succeeds in refraining from drinking, thirsty though they are. It follows from the premises stated that the human soul includes at least two distinct subjects, so that one opposite (the desire to drink) can be assigned to one of them and the other (the aversion to drinking) can be assigned to the other. Taking himself to have identified reason and appetite as distinct parts of the soul, Socrates draws attention to other kinds of conflict between desires, which are meant to bring to light spirit, the third part of the soul. 