in our power to assent or dissent to the like:  assent follows naturally, and consequently, properly speaking, is not subject to our command.  But some things which are apprehended do not convince the intellect to such an extent as not to leave it free to assent or dissent, or at least suspend its assent or dissent, on account of some cause or other; and in such things assent or dissent is in our power, and is subject to our command.

Reply Obj. 1:  Reason commands itself, just as the will moves itself, as stated above (Q. 9, A. 3), that is to say, in so far as each power reacts on its own acts, and from one thing tends to another.

Reply Obj. 2:  On account of the diversity of objects subject to the act of the reason, nothing prevents the reason from participating in itself:  thus the knowledge of principles is participated in the knowledge of the conclusions.

The reply to the third object is evident from what has been said.
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SEVENTH ARTICLE [I-II, Q. 17, Art. 7]

Whether the Act of the Sensitive Appetite Is Commanded?

Objection 1:  It would seem that the act of the sensitive appetite is not commanded.  For the Apostle says (Rom. 7:15):  “For I do not that good which I will”:  and a gloss explains this by saying that man lusts, although he wills not to lust.  But to lust is an act of the sensitive appetite.  Therefore the act of the sensitive appetite is not subject to our command.

Obj. 2:  Further, corporeal matter obeys God alone, to the effect of formal transmutation, as was shown in the First Part (Q. 65, A. 4; Q. 91, A. 2; Q. 110, A. 2).  But the act of the sensitive appetite is accompanied by a formal transmutation of the body, consisting in heat or cold.  Therefore the act of the sensitive appetite is not subject to man’s command.

Obj. 3:  Further, the proper motive principle of the sensitive appetite is something apprehended by sense or imagination.  But it is not always in our power to apprehend something by sense or imagination.  Therefore the act of the sensitive appetite is not subject to our command.

On the contrary, Gregory of Nyssa [Nemesius, De Nat.  Hom. xvi.] says:  “That which obeys reason is twofold, the concupiscible and the irascible,” which belong to the sensitive appetite.  Therefore the act of the sensitive appetite is subject to the command of reason.

I answer that, An act is subject to our command, in so far as it is in our power, as stated above (A. 5).  Consequently in order to understand in what manner the act of the sensitive appetite is subject to the command of reason, we must consider in what manner it is in our power.  Now it must be observed that the sensitive appetite differs from the intellective appetite, which is called the will, in the fact that the sensitive appetite is a power of a corporeal organ, whereas the will is not.  Again, every act of a power that uses a corporeal organ, depends not only on a power of the soul, but also on the disposition of that corporeal organ:  thus the act of vision depends on the power of sight, and on the condition of the eye, which condition is a help or a hindrance to that act.  Consequently the act of the sensitive appetite depends not only on the appetitive power, but also on the disposition of the body.