If God sees that the intellect has entirely submitted to Him and puts its hope in Him alone.

He strengthens it, saying: ‘Have no fear Jacob my son, my little Israel’ (Isa. 41:14. LXX), and:

‘Have no fear: for I have delivered you, I have called you by My name; you are Mine. If you pass through water, I shall be with you, and the rivers will not drown you.

‘If you go through fire, you will not be burnt, and the names will not consume you. For I am the Lord your God, the Holy One of Israel, who saves you’ (cf . Isa. 43:1-3. LXX).

When the intellect hears these words of reassurance, it says boldly to its enemies: ‘Who would fight with me? Let him stand against me. And who would accuse me?

‘Let him draw near to me. Behold, the Lord is my helper; who will harm me? Behold, all of you are like an old moth-eaten garment’ (cf Isa. 50:8-9. LXX).

If your heart comes to feel a natural hatred for sin, it has defeated the causes of sin and freed itself from them. Keep hell’s torments in mind: but know that your Helper is at hand.

Do nothing that will grieve Him, but say to Him with tears: ‘Be merciful and deliver me, Lord, for without Thy help I cannot escape from the hands of my enemies’.

Be attentive to your heart, and He will guard you from all evil. The monk should shut all the gates of his soul, that is, the senses, so that he is not lured astray.

When the intellect sees that it is not dominated by anything, it prepares itself for immortality, gathering its senses together and forming them into one body.

If your intellect is freed from all hope in things visible, this is a sign that sin has died in you. If your intellect is freed, the breach between it and God is eliminated.

If your intellect is freed from all its enemies and attains the Sabbath rest, it lives in another age, a new age in which it contemplates things new and undecaying.

For ‘wherever the dead body is, there will the eagles be gathered together’ (Matt. 24: 28).

The demons cunningly withdraw for a time in the hope that we will cease to guard our heart, thinking we have now attained peace, then they suddenly attack our unhappy soul and seize it like a sparrow.

[…]  Let us stand, therefore, with fear of God and keep guard over our heart, practicing the virtues which check the wickedness of our enemies.

Isaiah the Solitary (d. 489/491): On Guarding the Intellect, 4-11, 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).