Objection 1. It would seem that the act of the reason cannot be commanded. For it seems impossible for a thing to command itself. But it is the reason that commands, as stated above (Article 1). Therefore the act of the reason is not commanded.
Objection 2. Further, that which is essential is different from that which is by participation. But the power whose act is commanded by reason, is rational by participation, as stated in Ethic. i, 13. Therefore the act of that power, which is essentially rational, is not commanded.
Objection 3. Further, that act is commanded, which is in our power. But to know and judge the truth, which is the act of reason, is not always in our power. Therefore the act of the reason cannot be commanded.
On the contrary, That which we do of our free-will, can be done by our command. But the acts of the reason are accomplished through the free-will: for Damascene says (De Fide Orth. ii, 22) that "by his free-will man inquires, considers, judges, approves." Therefore the acts of the reason can be commanded.
I answer that, Since the reason reacts on itself, just as it directs the acts of other powers, so can it direct its own act. Consequently its act can be commanded.
But we must take note that the act of the reason may be considered in two ways. First, as to the exercise of the act. And considered thus, the act of the reason can always be commanded: as when one is told to be attentive, and to use one's reason. Secondly, as to the object; in respect of which two acts of the reason have to be noticed. One is the act whereby it apprehends the truth about something. This act is not in our power: because it happens in virtue of a natural or supernatural light. Consequently in this respect, the act of the reason is not in our power, and cannot be commanded. The other act of the reason is that whereby it assents to what it apprehends. If, therefore, that which the reason apprehends is such that it naturally assents thereto, e.g. the first principles, it is not 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 to Objection 1. Reason commands itself, just as the will moves itself, as stated above (Question 9, Article 3), that is to say, in so far as each power reacts on its own acts, and from one thing tends to another.
Reply to Objection 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.
Source: Summa Theologica (New Advent)