28 Here again arises a very difficult question. For in what way shall we fools be able to find a wise man, whereas this name, although hardly any one dare openly, yet most men lay claim to indirectly: so disagreeing one with another in the very matters, in the knowledge of which wisdom consists, as that it must needs be that either none of them, or but some certain one be wise? But when the fool enquires, who is that wise man? I do not at all see, in what way he can be distinguished and perceived.
For by no signs whatever can one recognize any thing, unless he shall have known that thing, whereof these are signs. But the fool is ignorant of wisdom. For not, as, in the case of gold and silver and other things of that kind, it is allowed both to know them when you see them and not to have them, thus may wisdom be seen by the mind's eye of him who has it not. For whatever things we come into contact with by bodily sense, are presented to us from without; and therefore we may perceive by the eyes what belong to others, when we ourselves possess not any of them or of that kind.
But what is perceived by the understanding is within in the mind, and to have it is nothing else than to see. But the fool is void of wisdom, therefore he knows not wisdom. For he could not see it with the eyes: but he cannot see it and not have it, nor have it and be a fool. Therefore he knows it not, and, so long as he knows it not, he cannot recognize it in another place. No one, so long as he is a fool, can by most sure knowledge find out a wise man, by obeying whom he may be set free from so great evil of folly.
Source: On the Profit of Believing (New Advent)