The saints tell us a great deal about watching our heart and mind, paying attention to both what goes into and comes out of them. St. Philotheos of Sinai describes the wonderful result that comes from such watchfulness:

Let us go forward with the heart completely attentive and the soul fully conscious. For if attentiveness and prayer are daily joined together, they become like Elijah’s fire-bearing chariot, raising us to heaven. What do I mean? A spiritual heaven with sun, moon and stars, is formed in the blessed heart of one who has reached a state of watchfulness, or who strives to attain it; for such a heart, as a result of mystical contemplation and ascent, is enabled to contain within itself the uncontainable God.