It is comparatively easy when he is only partially convinced.
He is partially convinced because he has found this or that proof of the thing, and he can expound it.

But a man is not really convinced of a philosophic theory when he finds that something proves it.

He is only really convinced when he finds that everything proves it.

And the more converging reasons he finds pointing to this conviction, the more bewildered he is if asked suddenly to sum them up.

Thus, if one asked an ordinary intelligent man, on the spur of the moment, "Why do you prefer civilisation to savagery ?" he would look wildly round at object after object, and would only be able to answer vaguely, "Why, there is that bookcase ...