One should explain in the following way, for example, breathing exists for the sake of this, while that comes to be from necessity because of these. But ‘necessity’ sometimes signifies that if that – that is, that for the sake of which – is to be, it is necessary for these things to obtain, while at other times it signifies that things are thus in respect of their character or nature. For it is necessary for the hot to go out and enter again upon meeting resistance, and for the air to flow in. This is directly necessary; and it is as the internal heat retreats during the cooling of the external air that inhalation and exhalation occur. This then is the manner of proper inquiry, and it is concerning these things and things of this sort that one should grasp the causes.