The Rhetor is one who does not teach (according to the logical process previously described), but persuades; guiding the mind by discourse to or from various opinions or sentiments.[102] Now if this is to be done _by art_ and methodically--that is, upon principle or system explicable and defensible--it pre-supposes (according to Plato) a knowledge of truth, and can only be performed by the logical expositor. For when men are deceived, it is only because they mistake what is like truth for truth itself: when they are undeceived, it is because they are made to perceive that what they believe to be truth is only an apparent likeness thereof. Such resemblances are strong or faint, differing by many gradations. Now no one can detect, or bring into account, or compare, these shades of resemblance, except he who knows the truth to which they all ultimately refer. It is through the slight differences that deception is operated. To deceive a man, you must carry him gradually away from the truth by transitional stages, each resembling that which immediately precedes, though the last in the series will hardly at all resemble the first: to undeceive him (or to avoid being deceived yourself), you must conduct him back by the counter-process from error to truth, by a series of transitional resemblances tending in that direction. You cannot do this like an artist (on system and by pre-determination), unless you know what the truth is.[103] By anyone who does not know, the process will be performed without art, or at haphazard. [Footnote 102: Plato, Phædrus, p. 261 A. [Greek: ê( r(êtorikê\ te/chnê psuchagôgi/a tis dia\ lo/gôn], &c.] [Footnote 103: Plato, Phædrus, pp. 262 A-D, 273 D.] [Side-note: He must then classify the minds to be persuaded, and the means of persuasion or varieties of discourse. He must know how to fit on the one to the other in each particular case.]