By St. Mark the Ascetic (Letter to Nicolas the Solitary)

Strive to enter the shrine within you and you will see the shrine of heaven, for the one is the same as the other and a single entrance permits you to contemplate both. The ladder leading to that kingdom is hidden within you, that is, within your soul: cleanse yourself from sin and there you will find the steps by which to ascend.

–St. Isaac the Syrian, Ascetical Homilies

If, my son, you wish to acquire within yourself your own lamp of noetic light and spiritual knowledge, so as to walk without stumbling in the dark night of this age; and if you wish your steps to be ordered by the Lord, delighting in the way of the Gospel – that is, desiring with ardent faith, with zeal and prayer, to practice the commandments of the Gospel – then I will show you a wonderful spiritual method to help you achieve this. It does not call for bodily exertion, but requires spiritual effort, control of the intellect, and an attentive understanding, assisted by fear and love of God. Through this method you can easily put to flight the cohorts of the enemy. If, then, you wish to triumph over the passions, enter within yourself through prayer and with the help of God. Descend into the depths of the heart, and search out the three powerful giants – forgetfulness, sloth and ignorance – which enable the rest of the evil passions to infiltrate into the self-indulgent soul, and to live, energize and flourish there. Then through strict attentiveness and control of the intellect, together with help from above, you will track down these evil giants, about which most people are ignorant; and so you will be able to free yourself from them by means of strict attentiveness and prayer. For when, through the action of grace, zeal for true knowledge, for mindfulness of God’s words and fore genuine concord is diligently planted and cultivated in the heart, then the last traces of forgetfulness, ignorance and sloth are expunged from it.