Objection 1. It would seem that teaching is a work not of the active but of the contemplative life. For Gregory says (Hom. v in Ezech.) that "the perfect who have been able to contemplate heavenly goods, at least through a glass, proclaim them to their brethren, whose minds they inflame with love for their hidden beauty." But this pertains to teaching. Therefore teaching is a work of the contemplative life.
Objection 2. Further, act and habit would seem to be referable to the same kind of life. Now teaching is an act of wisdom: for the Philosopher says (Metaph. i, 1) that "to be able to teach is an indication of knowledge." Therefore since wisdom or knowledge pertain to the contemplative life, it would seem that teaching also belongs to the contemplative life.
Objection 3. Further, prayer, no less than contemplation, is an act of the contemplative life. Now prayer, even when one prays for another, belongs to the contemplative life. Therefore it would seem that it belongs also to the contemplative life to acquaint another, by teaching him, of the truth we have meditated.
On the contrary, Gregory says (Hom. xiv in Ezech.): "The active life is to give bread to the hungry, to teach the ignorant the words of wisdom."
I answer that, The act of teaching has a twofold object. For teaching is conveyed by speech, and speech is the audible sign of the interior concept. Accordingly one object of teaching is the matter or object of the interior concept; and as to this object teaching belongs sometimes to the active, sometimes to the contemplative life. It belongs to the active life, when a man conceives a truth inwardly, so as to be directed thereby in his outward action; but it belongs to the contemplative life when a man conceives an intelligible truth, in the consideration and love whereof he delights. Hence Augustine says (De Verb. Dom. Serm. civ, 1): "Let them choose for themselves the better part," namely the contemplative life, "let them be busy with the word, long for the sweetness of teaching, occupy themselves with salutary knowledge," thus stating clearly that teaching belongs to the contemplative life.
The other object of teaching is on the part of the speech heard, and thus the object of teaching is the hearer. As to this object all doctrine belongs to the active life to which external actions pertain.
Reply to Objection 1. The authority quoted speaks expressly of doctrine as to its matter, in so far as it is concerned with the consideration and love of truth.
Reply to Objection 2. Habit and act have a common object. Hence this argument clearly considers the matter of the interior concept. For it pertains to the man having wisdom and knowledge to be able to teach, in so far as he is able to express his interior concept in words, so as to bring another man to understand the truth.
Reply to Objection 3. He who prays for another does nothing towards the man for whom he prays, but only towards God Who is the intelligible truth; whereas he who teaches another does something in his regard by external action. Hence the comparison fails.
Source: Summa Theologica (New Advent)