The Forms Good things are not the same as goodness (Form of the Good). –If all good things were destroyed, this wouldn’t destroy goodness itself. Forms don’t exist in any particular place or time. Forms don’t change. Forms are perfect examples (nothing is more good than goodness itself).
Sense experience All objects of experience are particular things. All particular things are both one thing, e.g. large, beautiful, good, and the opposite. If something is both X and not-X, then we can’t know that it is X.
Knowing the Forms The Form of beauty is beautiful under all conditions, to all observers, at all times. The Form of beauty is pure beauty; it (alone) is not both beautiful and not beautiful. Therefore, we can have knowledge of the Forms, though not through our senses. The highest knowledge is knowledge of the Form of the Good: it is from the good that ‘things that are just and so on derive their usefulness and value… Is there any point in having these other forms of knowledge without that of the good…?’ (505a-b)
The simile of the sun The visible worldThe intelligible world The sunThe Form of the Good The eyeThe mind (reason) SightIntelligence To seeTo know LightTruth GrowthThe being (reality) of the Forms
The Form of the Good Just as sun is the source of light and the source of sight, the Form of the Good ‘gives the objects of knowledge [the Forms] their truth and the knower’s mind the power of knowing’ (508a). Just as the sun is cause of growth, the Form of the Good is the source of the very being of knowable objects (509b) Reality is related to goodness: knowledge of what something is is knowledge of what it is for it to be a good example of its kind.
Virtue If you know goodness, you will be good. Just as an eye can’t be turned unless the whole body is turned, so the whole mind must be turned to be able to see the Forms. The love of wisdom subdues other desires (physical pleasure, greed, fear). The philosopher ‘assimilates’ himself to what he enjoys, viz. the Form of the Good.
Objection How does such abstract knowledge help practically? Plato never says. How does the philosopher become good? Why would knowledge alone make one good? Plato argues that philosophers are prone to corruption, influenced by praise. So philosophy only produces virtue in a virtuous society.