Isaiah the Solitary: “I entreat you not to leave your heart unguarded”
“I entreat you not to leave your heart unguarded, so long as you are in the body. Just as a farmer cannot feel confident about the crop growing in his fields, because he does not know what will happen to it before it is stored away in his granary, so a man should not leave his heart unguarded so long as he still has breath in his nostrils.
Up to his last breath he 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.”
St Isaiah the Solitary, On Guarding the Intellect, in the Philokalia