This is the last chapter of Book II.
That moral virtue is a mean, then, and in what sense it is so, and that it is a mean between two vices, the one involving excess, the other deficiency, and that it is such because its character is to aim at what is intermediate in passions and in actions, has been sufficiently stated. Hence also it is no easy task to be good. For in everything it is no easy task to find the middle, e.g. to find the middle of a circle is not for every one but for him who knows; so, too, any one can get angry- that is easy- or give or spend money; but to do this to the right person, to the right extent, at the right time, with the right motive, and in the right way, that is not for every one, nor is it easy; wherefore goodness is both rare and laudable and noble.
This is the point I was making in the update to the Bladesman's post. Because this is a pragmatic ethics, it is about adapting yourself to the world. It's not a purely internal exercise, where you determine how much courage or temperance or justice is right for your soul. You have to be right not just for yourself, but for the external situation. Sometimes more or less will be needed because the challenge you are facing is greater or lesser.
Hence he who aims at the intermediate must first depart from what is the more contrary to it, as Calypso advises-
Hold the ship out beyond that surf and spray.
Obviously that is the only way to push off in a ship, so the Homeric reference may seem superfluous. Unlike Plato, who was constantly at war with the poets, Aristotle has the greatest respect for Homer especially. It conveyed to his students, especially Alexander: "Aristotle taught Alexander and his companions about medicine, philosophy, morals, religion, logic, and art. Under Aristotle's tutelage, Alexander developed a passion for the works of Homer, and in particular the Iliad; Aristotle gave him an annotated copy, which Alexander later carried on his campaigns."
For of the extremes one is more erroneous, one less so; therefore, since to hit the mean is hard in the extreme, we must as a second best, as people say, take the least of the evils; and this will be done best in the way we describe. But we must consider the things towards which we ourselves also are easily carried away; for some of us tend to one thing, some to another; and this will be recognizable from the pleasure and the pain we feel. We must drag ourselves away to the contrary extreme; for we shall get into the intermediate state by drawing well away from error, as people do in straightening sticks that are bent.
This is a restatement of yesterday's point about love of pleasure being much more dangerous than insensibility to pleasure, for example. He continues on the subject:
Now in everything the pleasant or pleasure is most to be guarded against; for we do not judge it impartially. We ought, then, to feel towards pleasure as the elders of the people felt towards Helen, and in all circumstances repeat their saying; for if we dismiss pleasure thus we are less likely to go astray. It is by doing this, then, (to sum the matter up) that we shall best be able to hit the mean.
The reference is to Iliad 3: "The two sages, Ucalegon and Antenor, elders of the people, were seated by the Scaean gates, with Priam, Panthous, Thymoetes, Lampus, Clytius, and Hiketaon of the race of Mars. These were too old to fight, but they were fluent orators, and sat on the tower like cicales that chirrup delicately from the boughs of some high tree in a wood. When they saw Helen coming towards the tower, they said softly to one another, 'Small wonder that Trojans and Achaeans should endure so much and so long, for the sake of a woman so marvellously and divinely lovely. Still, fair though she be, let them take her and go, or she will breed sorrow for us and for our children after us.'"
But this is no doubt difficult, and especially in individual cases; for or is not easy to determine both how and with whom and on what provocation and how long one should be angry; for we too sometimes praise those who fall short and call them good-tempered, but sometimes we praise those who get angry and call them manly. The man, however, who deviates little from goodness is not blamed, whether he do so in the direction of the more or of the less, but only the man who deviates more widely; for he does not fail to be noticed. But up to what point and to what extent a man must deviate before he becomes blameworthy it is not easy to determine by reasoning, any more than anything else that is perceived by the senses; such things depend on particular facts, and the decision rests with perception. So much, then, is plain, that the intermediate state is in all things to be praised, but that we must incline sometimes towards the excess, sometimes towards the deficiency; for so shall we most easily hit the mean and what is right.
That's fair enough. This is hard work, and our reason can only get us so far sometimes given the depths of emotional entanglement and the difficulty of the problems. Sometimes we'll go astray, but if we err in the wisest direction we'll be closer to what was best.