Theophylact of Ohrid, on Titus 1:15

“All things are clean to the clean: but to them that are defiled, and to unbelievers, nothing is clean: but both their mind and their conscience are defiled.”

“So then, foods are not clean or unclean by their own nature, but by the disposition of those who partake of them. The latter, being clean and pious, know that all things are clean, as the creation of God, and that only sin is unclean. For even if the law considered certain things unclean, this was not without purpose, but for the restraint of intemperance, knowing that the Jews would not have obeyed it without compulsion, being gluttons. Of course, if those who partake are clean and pious, then for such people all things will be clean. In what way? For if one reasons thus, then fish that devour men, and birds considered clean but feeding on worms, ought to seem unclean. Therefore an unclean mind, directed toward the bad side, by itself defiles what is not such by nature. In the same way, one suffering from a stomach ailment thinks that foods are unpleasant, even though they may be pleasant; and to one suffering from dizziness, the firmly standing earth seems to be moving. Such suspicion depends on his illness. This applies both to the Manichaeans and to the Marcionites and to the heretics newly sprung from them, called by most people Galatae.”

Imported from an open dataset — not yet checked against the printed edition.