Saint Theognostos. "On the Practice of the Virtues, Contemplation and the Priesthood". From Philokalia, Vol. 2.
Do not try to embark on the higher forms of contemplation before you have achieved complete dispassion, and do not pursue what lies as yet beyond your reach. If your wish is to become a theologian and a contemplative, ascend by the path of ascetic practice and through self-purification acquire what is pure. Do not pursue theology beyond the limits of your present state of development: it is wrong for us who are still drinking the milk of the virtues to attempt to soar to the heights of theology, and if we do so we will flounder like fledglings, however great the longing roused within us by the honey of spiritual knowledge. But, once purified by self-restraint and tears, we will be lifted up from the earth like Elijah or Habakkuk (cf. 2 Kgs. 2:11; Bel and Dr., verses 36-39), anticipating the moment when we will be caught up into the clouds (cf. 1 Thess. 4:17); and transported beyond the world of the senses by undistracted prayer, pure and contemplative, we may then in our search for God touch the fringe of theology.
7. If you wish to be granted a mental vision of the divine you must first embrace a peaceful and quiet way of life, and devote your efforts to acquiring a knowledge of both yourself and God. If you do this and achieve a pure state untroubled by any passion, there, is nothing to prevent your intellect from perceiving, as it were in a light breeze (cf. 1 Kgs. 19:12 LXX), Him who is invisible to all; and He will bring you good tidings of salvation through a yet clearer knowledge of Himself.