by St. Basil the Great
Physical ascesis is determined by each person according to his physical strength. I think it is proper to take care not to undermine the body’s strength by excessive abstinence and thus render it useless and ineffective for profitable activity. The body should be active and by no means disabled by excess.
Had it been beneficial for us that our body be disabled and prostrate, as if dead and scarcely breathing, then surely God would have created us this way in the beginning. But since He did not create us this way, then those people sin who do not preserve intact what was created good.
The devout ascetic should concern himself with only one thing: Is there any evil concealed in his soul because of laziness or sloth? Have sobriety and the earnest striving of his mind toward God at any point been weakened? Have his spiritual sanctification and the resulting enlightenment of his soul in any way been darkened? For if all the above virtues keep growing within him, there will be no time at all for the bodily passions to arise, since his soul is occupied with things on high and leaves the body no time for the passions to arise. If the soul is thus disposed, the person who partakes of food is not different from the one who does not eat at all.
Such a person has mastered not only fasting but total abstinence as well, and he is to be commended for his excellent discernment concerning his body. A temperate life does not know the burning of desire.