Objection 1. It would seem that there are not several powers of the soul. For the intellectual soul approaches nearest to the likeness of God. But in God there is one simple power: and therefore also in the intellectual soul.
Objection 2. Further, the higher a power is, the more unified it is. But the intellectual soul excels all other forms in power. Therefore above all others it has one virtue or power.
Objection 3. Further, to operate belongs to what is in act. But by the one essence of the soul, man has actual existence in the different degrees of perfection, as we have seen above (76, 3,4). Therefore by the one power of the soul he performs operations of various degrees.
On the contrary, The Philosopher places several powers in the soul (De Anima ii, 2,3).
I answer that, Of necessity we must place several powers in the soul. To make this evident, we observe that, as the Philosopher says (De Coelo ii, 12), the lowest order of things cannot acquire perfect goodness, but they acquire a certain imperfect goodness, by few movements; and those which belong to a higher order acquire perfect goodness by many movements; and those yet higher acquire perfect goodness by few movements; and the highest perfection is found in those things which acquire perfect goodness without any movement whatever. Thus he is least of all disposed of health, who can only acquire imperfect health by means of a few remedies; better disposed is he who can acquire perfect health by means of many remedies; and better still, he who can by few remedies; best of all is he who has perfect health without any remedies. We conclude, therefore, that things which are below man acquire a certain limited goodness; and so they have a few determinate operations and powers. But man can acquire universal and perfect goodness, because he can acquire beatitude. Yet he is in the last degree, according to his nature, of those to whom beatitude is possible; therefore the human soul requires many and various operations and powers. But to angels a smaller variety of powers is sufficient. In God there is no power or action beyond His own Essence.
There is yet another reason why the human soul abounds in a variety of powers--because it is on the confines of spiritual and corporeal creatures; and therefore the powers of both meet together in the soul.
Reply to Objection 1. The intellectual soul approaches to the Divine likeness, more than inferior creatures, in being able to acquire perfect goodness; although by many and various means; and in this it falls short of more perfect creatures.
Reply to Objection 2. A unified power is superior if it extends to equal things: but a multiform power is superior to it, if it is over many things.
Reply to Objection 3. One thing has one substantial existence, but may have several operations. So there is one essence of the soul, with several powers.
Source: Summa Theologica (New Advent)