13. Let it not then add anything to that which it knows itself to be, when it is bidden to know itself. For it knows, at any rate, that this is said to itself; namely, to the self that is, and that lives, and that understands. But a dead body also is, and cattle live; but neither a dead body nor cattle understand. Therefore it so knows that it so is, and that it so lives, as an understanding is and lives. When, therefore, for example's sake, the mind thinks itself air, it thinks that air understands; it knows, however, that itself understands, but it does not know itself to be air, but only thinks so. Let it separate that which it thinks itself; let it discern that which it knows; let this remain to it, about which not even have they doubted who have thought the mind to be this corporeal thing or that. For certainly every mind does not consider itself to be air; but some think themselves fire, others the brain, and some one kind of corporeal thing, others another, as I have mentioned before; yet all know that they themselves understand, and are, and live; but they refer understanding to that which they understand, but to be, and to live, to themselves. And no one doubts, either that no one understands who does not live, or that no one lives of whom it is not true that he is; and that therefore by consequence that which understands both is and lives; not as a dead body is which does not live, nor as a soul lives which does not understand, but in some proper and more excellent manner. Further, they know that they will, and they equally know that no one can will who is not and who does not live; and they also refer that will itself to something which they will with that will. They know also that they remember; and they know at the same time that nobody could remember, unless he both was and lived; but we refer memory itself also to something, in that we remember those things. Therefore the knowledge and science of many things are contained in two of these three, memory and understanding; but will must be present, that we may enjoy or use them. For we enjoy things known, in which things themselves the will finds delight for their own sake, and so reposes; but we use those things, which we refer to some other thing which we are to enjoy. Neither is the life of man vicious and culpable in any other way, than as wrongly using and wrongly enjoying. But it is no place here to discuss this.
14. But since we treat of the nature of the mind, let us remove from our consideration all knowledge which is received from without, through the senses of the body; and attend more carefully to the position which we have laid down, that all minds know and are certain concerning themselves. For men certainly have doubted whether the power of living, of remembering, of understanding, of willing, of thinking, of knowing, of judging, be of air, or of fire, or of the brain, or of the blood, or of atoms, or besides the usual four elements of a fifth kind of body, I know not what; or ,whether the combining or tempering together of this our flesh itself has power to accomplish these things. And one has attempted to establish this, and another to establish that. Yet who ever doubts that he himself lives, and remembers, and understands, and wills, and thinks, and knows, and judges? Seeing that even if he doubts, he lives; if he doubts, he remembers why he doubts; if he doubts, he understands that he doubts; if he doubts, he wishes to be certain; if he doubts, he thinks; if he doubts, he knows that he does not know; if he doubts, he judges that he ought not to assent rashly. Whosoever therefore doubts about anything else, ought not to doubt of all these things; which if they were not, he would not be able to doubt of anything.
15. They who think the mind to be either a body or the combination or tempering of the body, will have all these things to seem to be in a subject, so that the substance is air, or fire, or some other corporeal thing, which they think to be the mind; but that the understanding (intelligentia) is in this corporeal thing as its quality, so that this corporeal thing is the subject, but the understanding is in the subject: viz. that the mind is the subject, which they judge to be a corporeal thing, but the understanding [intelligence], or any other of those things which we have mentioned as certain to us, is in that subject. They also hold nearly the same opinion who deny the mind itself to be body, but think it to be the combination or tempering together of the body; for there is this difference, that the former say that the mind itself is the substance, in which the understanding [intelligence] is, as in a subject; but the latter say that the mind itself is in a subject, viz. in the body, of which it is the combination or tempering together. And hence, by consequence, what else can they think, except that the understanding also is in the same body as in a subject?
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.