(Another way of trying to understand Plato's theory of Forms is to see Justice as a Form, Goodness as a Form, Happiness as a Form, even Size as a Form. If a man looks at something printed, it appears to be so small that he cannot read it. If he then applies a magnifying glass to it, it appears to be larger, and he can read it. But its Form [Size] has not changed.)
But the whole point of this aspect of the dialogue is to define the philosopher and to defend his credentials as a potential ruler. It is the philosopher who possesses the knowledge of the real; it is he who possesses the knowledge of the Forms as absolutes. (Plato is convinced that they are absolutes.) Justice, Goodness, Happiness, the Moral Life — all are absolutes; they may be perceived in their Forms; they are not relative to the times or the changing tides of political favoritism or animosities or "taste" or any sort of the "appearance or belief in appearances." Thus it is that philosophers should be kings. They are best qualified to rule.
As for the Dionysiacs to whom Glaucon referred earlier, and as to current politicians (in Plato's own times), they seem to be passionately involved in their belief in appearances. And their beliefs are always evanescent (fleeting and simply reflective of any given time in the life of mankind). These people are in fact simply amateurs in aesthetics and in statecraft, always followers, never leaders.
