St Thomas did not write a special treatise on mystical theology. In the golden age of Scholasticism there were no hard and fast divisions between systematic, moral, ascetical and mystical theology; but systematic theology furnished the rules by which to deal with the practical questions of the spiritual life. And so the Angelic Doctor gives us the principles of mystical theology, embedded in the Prima Secundæ of his Summa Theologica, which deals with man, made in the image of God, and with his virtues and vices in general (especially question 68, on the Gifts of the Holy Ghost) and in the Secunda Secundæ, which treats of particular virtues and states of life (especially questions 180 to 182, on the Contemplative and Active Life) and in his whole teaching on Grace and the Virtues.

The mystical life is nothing less than the life of grace lived at its highest level.