Our teacher Jesus Christ, out of pity for mankind and knowing the utter mercilessness of the demons, severely commands us: ‘Be ready at every hour, for you do not know when the thief will come; do not let him come and find you asleep’ (cf Matt. 24:42-43).
[...] Stand guard, then, over your heart and keep a watch on your senses; and if the remembrance of God dwells peaceably within you, you will catch the thieves when they try to deprive you of it.
When a man has an exact knowledge about the nature of thoughts, he recognizes those which are about to enter and defile him, troubling the intellect with distractions and making it lazy.
Those who recognize these evil thoughts for what they are remain undisturbed and continue in prayer to God.
[...] What…is meant by the worship of God?
It means that we have nothing extraneous in our intellect when we are praying to Him: neither sensual pleasure as we bless Him, nor malice as we sing His praise, nor hatred as we exalt Him, nor jealousy to hinder us as we speak to Him and call Him to mind.
For all these things are full of darkness; they are a wall imprisoning our wretched soul, and if the soul has them in itself it cannot worship God with purity.
They obstruct its ascent and prevent it from meeting God: they hinder it from blessing Him inwardly and praying to Him with sweetness of heart, and so receiving His illumination.
As a result the intellect is always shrouded in darkness and cannot advance in holiness, because it does not make the effort to uproot these thoughts by means of spiritual knowledge.
When the intellect rescues the soul’s senses from the desires of the flesh and imbues them with dispassion, the passions shamelessly attack the soul, trying to hold its senses fast in sin; but if the intellect then continually calls upon God in secret.
He, seeing all this, will send His help and destroy all the passions at once.
I entreat you not to leave your heart unguarded, so long as you are in the body.
[...] Up to his last breath [a man] cannot know what passion will attack him; so long as he breathes, therefore, he must not leave his heart unguarded, but should at every moment pray to God for His help and mercy.
Isaiah the Solitary (d. 489/491): On Guarding the Intellect, 12-15, Text from G.E.H. Palmer, Philip Sherrard, and Kallistos Ware (trans. and eds.) The Philokalia: The Complete Text, vol. I (Faber & Faber, London & Boston: 1979).
Note: The word intellect in the Philokalia translates the Greek nous, which the translators define as follows:
the highest faculty in man, through which – provided it is purified – he knows God or the inner essences or principles of created things by means of direct apprehension or spiritual perception. Unlike the dianoia or reason, from which it must be carefully distinguished, the intellect does not function by formulating abstract concepts and then arguing on this basis to a conclusion reached through deductive reasoning, but it understands divine truth by means of immediate experience, intuition or ‘simple cognition’ (the term used by St Isaac the Syrian). The intellect dwells in the ‘depths of the soul’; it constitutes the innermost aspect of the heart (St Diadochos). The intellect is the organ of contemplation, the ‘eye of the heart’ (Makarian Homilies).