Objection 1. It would seem that the contemplative life is hindered by the active life. For the contemplative life requires a certain stillness of mind, according to Psalm 45:11, "Be still, and see that I am God"; whereas the active life involves restlessness, according to (Luke 10:41), "Martha, Martha, thou art careful and troubled about many things." Therefore the active life hinders the contemplative.
Objection 2. Further, clearness of vision is a requisite for the contemplative life. Now active life is a hindrance to clear vision; for Gregory says (Hom. xiv in Ezech.) that it "is blear-eyed and fruitful, because the active life, being occupied with work, sees less." Therefore the active life hinders the contemplative.
Objection 3. Further, one contrary hinders the other. Now the active and the contemplative life are apparently contrary to one another, since the active life is busy about many things, while the contemplative life attends to the contemplation of one; wherefore they differ in opposition to one another. Therefore it would seem that the contemplative life is hindered by the active.
On the contrary, Gregory says (Moral. vi, 37): "Those who wish to hold the fortress of contemplation, must first of all train in the camp of action."
I answer that, The active life may be considered from two points of view. First, as regards the attention to and practice of external works: and thus it is evident that the active life hinders the contemplative, in so far as it is impossible for one to be busy with external action, and at the same time give oneself to Divine contemplation. Secondly, active life may be considered as quieting and directing the internal passions of the soul; and from this point of view the active life is a help to the contemplative, since the latter is hindered by the inordinateness of the internal passions. Hence Gregory says (Moral. vi, 37): "Those who wish to hold the fortress of contemplation must first of all train in the camp of action. Thus after careful study they will learn whether they no longer wrong their neighbor, whether they bear with equanimity the wrongs their neighbors do to them, whether their soul is neither overcome with joy in the presence of temporal goods, nor cast down with too great a sorrow when those goods are withdrawn. On this way they will known when they withdraw within themselves, in order to explore spiritual things, whether they no longer carry with them the shadows of the things corporeal, or, if these follow them, whether they prudently drive them away." Hence the work of the active life conduces to the contemplative, by quelling the interior passions which give rise to the phantasms whereby contemplation is hindered.
This suffices for the Replies to the Objections; for these arguments consider the occupation itself of external actions, and not the effect which is the quelling of the passions.
Source: Summa Theologica (New Advent)