“Sincerity and exactness will be required of us not just in our deeds but in our words, and not just in our words but in our thoughts. For He says, ‘If thine eye,’ meaning your innermost thought, ‘be evil, thy whole body shall be evil’ (cf. Matt 6:23). He uses the word ‘eye’ in the singular to show that He is referring to the mind, for the body doe not have just one eye.
He gives further proof of His meaning by going on to say, ‘Thy whole body shall be evil’ (Matt 6:23). Given that there are two eyes in the body, if one of them cannot see, the whole body is not afflicted with blindness. Also by saying that is shall be evil, rather than that it shall suffer, He makes it clear that He is speaking about the sickness of passions within the soul when we sin. That is why He adds, ‘If the light that is in thee,’ namely, your mind and your understanding, ‘be darkness, How great is that darkness!’ (Matt 6:23), meaning of the body and senses.
Indicating the cure for such secret ills, He commands us to pray to ‘the Father which is in secret’ (Matt 6:6), to give alms in secret and to cleanse rather the inside of the cup (cf Matt 23:26), which is our inner man.”
+ St. Gregory Palamas, Homily 54, On the Last Judgement