16. And all these do not perceive that the mind knows itself, even when it seeks for itself, as we have already shown. But nothing is at all rightly said to be known while its substance is not known. And therefore, when the mind knows itself, it knows its own substance; and when it is certain about itself, it as certain about its own substance. But it is certain about itself, as those things which are said above prove convincingly; although it is not at all certain whether itself is air, or fire, or some body, or some function of body. Therefore it is not any of these. And to that whole which is bidden to know itself, belongs this, that it is certain that it is not any of those things of which it is uncertain, and is certain that it is that only, which only it is certain that it is. For it thinks in this way of fire, or air, and whatever else of the body it thinks of. Neither can it in any way be brought to pass that it should so think that which itself is, as it thinks that which itself is not. Since it thinks all these things through an imaginary phantasy, whether fire, or air, or this or that body, or that part or combination and tempering together of the body: nor assuredly is it said to be all those things, but some one of them. But if it were any one of them, it would think this one in a different manner from the rest viz. not through an imaginary phantasy, as absent things are thought, which either themselves or some of like kind have been touched by the bodily sense; but by some inward, not feigned, but true presence (for nothing is more present to it than itself); just as it thinks that itself lives, and remembers, and understands, and wills. For it knows these things in itself, and does not imagine them as though it had touched them by the sense outside itself, as corporeal things are touched. And if it attaches nothing to itself from the thought of these things, so as to think itself to be something of the kind, then whatsoever remains to it from itself that alone is itself.