MENON may be considered as pre-eminently ethical in its design.  It is expressly devoted to the question—­Is Virtue teachable?  Sokrates as usual confesses that he does not know what virtue is.  He will not accept a catalogue of the admitted virtues as a definition of virtue, and presses for some common, or defining attribute.  He advances on his own side his usual doctrine that virtue is Knowledge, or a mode of Knowledge, and that it is good and profitable; which is merely an iteration of the Science of good and evil.  He distinguishes virtue from Right Opinion, a sort of quasi-knowledge, the knowledge of esteemed and useful citizens, which cannot be the highest knowledge, since these citizens fail to impart it even to their own sons.

In this dialogue, we have Plato’s view of Immortality, which comprises both pre-existence and post-existence.  The pre-existence is used to explain the derivation of general notions, or Ideas, which are antecedent to the perceptions of sense.

In PROTAGORAS, we find one of the most important of the ethical discussions of Plato.  It proceeds from the same question—­Is virtue teachable?—­Sokrates as usual expressing his doubts on the point.  Protagoras then delivers a splendid harangue, showing how virtue is taught—­namely, by the practice of society in approving, condemning, rewarding, punishing the actions of individuals.  From childhood upward, every human being in society is a witness to the moral procedure of society, and by degrees both knows, and conforms to, the maxims of virtue of the society.  Protagoras himself as a professed teacher, or sophist, can improve but little upon, this habitual inculcation.  Sokrates, at the end of the harangue, puts in his usual questions tending to bring out the essence or definition of virtue, and soon drives Protagoras into a corner, bringing him to admit a view nowhere else developed in Plato, that Pleasure is the only good, Pain the only evil, and that the science of Good and Evil consists in Measuring, and in choosing between conflicting pleasures and pains—­preferring the greater pleasure to the less, the less pain to the greater.  For example, courage is a wise estimate of things terrible and things not terrible.  In consistency with the doctrine that Knowledge is virtue, it is maintained here as elsewhere, that a man knowing good and evil must act upon that knowledge.  Plato often repeats his theory of Measurement, but never again specifically intimates that the things to be measured are pleasures and pains.  And neither here nor elsewhere, does he suppose the virtuous man taking directly into his calculation the pleasures and pains of other persons.