Why does St. Basil the Great insist on the prayer of the mind, especially for monastics?

St. Basil wants the monk to attain to the vision of God and union with Him. Then the monk’s life — despite the intrusions of work and toil — is overshadowed by the Holy Spirit, hovering over him every minute and every hour. Then prayer becomes the air he breathes and the food that nourishes him. Theophan the Recluse says, “I remember that St. Basil the Great solved the question how the Apostles could pray without ceasing, in this way: in everything they did, he replied, they thought of God and lived in constant devotion to Him. This spiritual state was their unceasing prayer.”²⁷¹