The Way of Perfection, by Teresa of Avila (1515-1582)
Explains the meaning of mental prayer.
The reason we sometimes do so (address God carelessly) is to be found in the humility of this King, Who, unskilled though I am in speaking with Him, does not refuse to hear me or forbid me to approach Him, or command His guards to throw me out. For the angels in His presence know well that their King is such that He prefers the unskilled language of a humble peasant boy, knowing that he would say more if he had more to say, to the speech of the wisest and most learned men, however elegant may be their arguments, if these are not accompanied by humility. But we must not be unmannerly because He is good. If only to show our gratitude to Him for enduring our foul odour and allowing such a one as myself to come near Him, it is well that we should try to realize His purity and His nature. It is true that we recognize this at once when we approach Him, just as we do when we visit the lords of the earth. Once we are told about their fathers' names and their incomes and dignities, there is no more for us to know about them; for on earth one makes account of persons, and honours them, not because of their merits but because of their possessions.
- Chapter 22
(Keep in mind that she is writing to sisters in a cloistered contemplative order.)