The "ladder of love" occurs in the text Symposium (c. 385-370 BC) by the ancient Greek philosopher Plato. It's about a contest at a men's banquet, involving impromptu philosophical speeches in praise of Eros, the Greek god of love and sexual desire. Socrates summarized the speeches of five of the guests and then recounted the teachings of a priestess, Diotima. The ladder is a metaphor for the ascent a lover might make from purely physical attraction to something beautiful, as a beautiful body, the lowest rung, to actual contemplation of the Form of Beauty itself.
Diotima maps out the stages in this ascent in terms of what sort of beautiful thing the lover desires and is drawn toward.
- A particular beautiful body. This is the starting point, when love, which by definition is a desire for something we don't have, is first aroused by the sight of individual beauty.
- All beautiful bodies. According to standard Platonic doctrine, all beautiful bodies share something in common, something the lover eventually comes to recognize. When he does recognize this, he moves beyond a passion for any particular body.
- Beautiful souls. Next, the lover comes to realize that spiritual and moral beauty matters much more than physical beauty. So he will now yearn for the sort of interaction with noble characters that will help him become a better person.
- Beautiful laws and institutions. These are created by good people (beautiful souls) and are the conditions which foster moral beauty.
- The beauty of knowledge. The lover turns his attention to all kinds of knowledge, but particularly, in the end to philosophical understanding. (Although the reason for this turn isn't stated, it is presumably because philosophical wisdom is what underpins good laws and institutions.)
- Beauty itself - that is, the Form of the Beautiful. This is described as "an everlasting loveliness which neither comes nor goes, which neither flowers nor fades." It is the very essence of beauty, "subsisting of itself and by itself in an eternal oneness." And every particular beautiful thing is beautiful because of its connection to this Form. The lover who has ascended the ladder apprehends the Form of Beauty in a kind of vision or revelation, not through words or in the way that other sorts of more ordinary knowledge are known.
Diotima tells Socrates that if he ever reached the highest rung on the ladder and contemplated the Form of Beauty, he would never again be seduced by the physical attractions of beautiful youths. Nothing could make life more worth living than enjoying this sort of vision. Because the Form of Beauty is perfect, it will inspire perfect virtue in those who contemplate it.
This account of the ladder of love is the source for the familiar notion of "Platonic love," by which is meant the sort of love that is not expressed through sexual relations. The description of the ascent can be viewed as an account of sublimation, the process of transforming one sort of impulse into another, usually, one that is viewed as "higher" or more valuable. In this instance, the sexual desire for a beautiful body becomes sublimated into a desire for philosophical understanding and insight.