Objection 1. It would seem that there is both an active and a passive intellect in an angel. The Philosopher says (De Anima iii, text. 17) that, "in the soul, just as in every nature, there is something whereby it can become all things, and there is something whereby it can make all things." But an angel is a kind of nature. Therefore there is an active and a passive intellect in an angel.
Objection 2. Further, the proper function of the passive intellect is to receive; whereas to enlighten is the proper function of the active intellect, as is made clear in De Anima iii, text. 2,3,18. But an angel receives enlightenment from a higher angel, and enlightens a lower one. Therefore there is in him an active and a passive intellect.
On the contrary, The distinction of active and passive intellect in us is in relation to the phantasms, which are compared to the passive intellect as colors to the sight; but to the active intellect as colors to the light, as is clear from De Anima iii, text. 18. But this is not so in the angel. Therefore there is no active and passive intellect in the angel.
I answer that, The necessity for admitting a passive intellect in us is derived from the fact that we understand sometimes only in potentiality, and not actually. Hence there must exist some power, which, previous to the act of understanding, is in potentiality to intelligible things, but which becomes actuated in their regard when it apprehends them, and still more when it reflects upon them. This is the power which is denominated the passive intellect. The necessity for admitting an active intellect is due to this--that the natures of the material things which we understand do not exist outside the soul, as immaterial and actually intelligible, but are only intelligible in potentiality so long as they are outside the soul. Consequently it is necessary that there should be some power capable of rendering such natures actually intelligible: and this power in us is called the active intellect.
But each of these necessities is absent from the angels. They are neither sometimes understanding only in potentiality, with regard to such things as they naturally apprehend; nor, again, are their intelligible objects in potentiality, but they are actually such; for they first and principally understand immaterial things, as will appear later (84, 7; 85, 1). Therefore there cannot be an active and a passive intellect in them, except equivocally.
Reply to Objection 1. As the words themselves show, the Philosopher understands those two things to be in every nature in which there chances to be generation or making. Knowledge, however, is not generated in the angels, but is present naturally. Hence there is not need for admitting an active and a passive intellect in them.
Reply to Objection 2. It is the function of the active intellect to enlighten, not another intellect, but things which are intelligible in potentiality, in so far as by abstraction it makes them to be actually intelligible. It belongs to the passive intellect to be in potentiality with regard to things which are naturally capable of being known, and sometimes to apprehend them actually. Hence for one angel to enlighten another does not belong to the notion of an active intellect: neither does it belong to the passive intellect for the angel to be enlightened with regard to supernatural mysteries, to the knowledge of which he is sometimes in potentiality. But if anyone wishes to call these by the names of active and passive intellect, he will then be speaking equivocally; and it is not about names that we need trouble.
Source: Summa Theologica (New Advent)