Returning to the discussion of human friendships, there seems to be an agreement that relationships must be entered with caution. Aristotle says that "one should examine at the beginning by whom the good deed is done and what his conditions are, so that one can accept it on these conditions or reject it," and Cicero speaks of similar criteria: "We must test and observe first, and then bestow our affections..." These two writers promote caution because the motives of others are not always as noble as they are appear to be. Montaigne promotes a more moderate level of caution when he says that only the "common friendships" require careful examination: According to him, the true friendship can be trusted absolutely, while one "...must walk in those other friendships bridle in hand, with prudence and precaution; the knot is not so well tied that there is no cause to mistrust it." Saint Augustine also realizes that friends are often not what they seem: "In our present wretched condition we frequently mistake a friend for an enemy, and an enemy for a friend." He ascribes the deceptive quality of people to man's fallen nature, as Herbert A. Deane points out:

We must always remember that when [Augustine] says that 'the laws of men's nature move him to hold fellowship and maintain peace with all men so far as in him lies,' he is talking about the natural state of man before the Fall and the introduction into the world of sin.... Augustine's comment on the hatred and conflict that rage among men is bitterly sorrowful: 'For there is nothing so social by nature, so unsocial by its corruption, as this race....'
Only among the small number of men who have been redeemed by God's grace do we find the true unity and concord that are natural to man.

From a Christian perspective, men are untrustworthy as a result of original sin. In short, the fall of mankind has hindered our ability to engage in pure friendship.