Augustine of Hippo, On the Holy Trinity XII.10: The Lowest Degradation Reached by Degrees

15. When the soul then consults either for itself or for others with a good will towards perceiving the inner and higher things, such as are possessed in a chaste embrace, without any narrowness or envy, not individually, but in common by all who love such things; then even if it be deceived in anything, through ignorance of things temporal (for its action in this case is a temporal one), and if it does not hold fast to that mode of acting which it ought, the temptation is but one common to man. And it is a great thing so to pass through this life, on which we travel, as it were, like a road on our return home, that no temptation may take us, but what is common to man. For this is a sin, without the body, and must not be reckoned fornication, and on that account is very easily pardoned. But when the soul does anything in order to attain those things which are perceived through the body, through lust of proving or of surpassing or of handling them, in order that it may place in them its final good, then whatever it does, it does wickedly, and commits fornication, sinning against its own body: and while snatching from within the deceitful images of corporeal things, and combining them by vain thought, so that nothing seems to it to be divine, unless it be of such a kind as this; by selfish greediness it is made fruitful in errors, and by selfish prodigality it is emptied of strength. Yet it would not leap on at once from the commencement to such shameless and miserable fornication, but, as it is written, “He that contemneth small things, shall fall by little and little.”