Now whatever part the power of the soul takes in the act, follows apprehension.  And the apprehension of the imagination, being a particular apprehension, is regulated by the apprehension of reason, which is universal; just as a particular active power is regulated by a universal active power.  Consequently in this respect the act of the sensitive appetite is subject to the command of reason.  On the other hand, condition or disposition of the body is not subject to the command of reason:  and consequently in this respect, the movement of the sensitive appetite is hindered from being wholly subject to the command of reason.

Moreover it happens sometimes that the movement of the sensitive appetite is aroused suddenly in consequence of an apprehension of the imagination of sense.  And then such movement occurs without the command of reason:  although reason could have prevented it, had it foreseen.  Hence the Philosopher says (Polit. i, 2) that the reason governs the irascible and concupiscible not by a “despotic supremacy,” which is that of a master over his slave; but by a “politic and royal supremacy,” whereby the free are governed, who are not wholly subject to command.

Reply Obj. 1:  That man lusts, although he wills not to lust, is due to a disposition of the body, whereby the sensitive appetite is hindered from perfect compliance with the command of reason.  Hence the Apostle adds (Rom. 7:15):  “I see another law in my members, fighting against the law of my mind.”  This may also happen through a sudden movement of concupiscence, as stated above.

Reply Obj. 2:  The condition of the body stands in a twofold relation to the act of the sensitive appetite.  First, as preceding it:  thus a man may be disposed in one way or another, in respect of his body, to this or that passion.  Secondly, as consequent to it:  thus a man becomes heated through anger.  Now the condition that precedes, is not subject to the command of reason:  since it is due either to nature, or to some previous movement, which cannot cease at once.  But the condition that is consequent, follows the command of reason:  since it results from the local movement of the heart, which has various movements according to the various acts of the sensitive appetite.

Reply Obj. 3:  Since the external sensible is necessary for the apprehension of the senses, it is not in our power to apprehend anything by the senses, unless the sensible be present; which presence of the sensible is not always in our power.  For it is then that man can use his senses if he will so to do; unless there be some obstacle on the part of the organ.  On the other hand, the apprehension of the imagination is subject to the ordering of reason, in proportion to the strength or weakness of the imaginative power.  For that man is unable to imagine the things that reason considers, is either because they cannot be imagined, such as incorporeal things; or because of the weakness of the imaginative power, due to some organic indisposition. ________________________