written on the same subjects; but who they are, is more than they know themselves. Thus much at least, I can say about all writers, past or future, who say they know the things to which I devote myself, whether by hearing the teaching of me or of others, or by their own discoveries-that according to my view it is not possible for them to have any real skill in the matter. There neither is nor ever will be a treatise of mine on the subject. For it does not admit of exposition like other branches of knowledge; but after much converse about the matter itself and a life lived together, suddenly a light, as it were, is kindled in one soul by a flame that leaps to it from another, and thereafter sustains itself. 去书内