The ascent begins with what is most familiar — the beauty of a single beautiful body. But Diotima insists this is only the first rung. Once the lover sees that the beauty of one form is akin to the beauty of every other, he stops being the slave of one person and becomes a lover of all beautiful forms. From bodies he rises to souls, recognising that the beauty of a noble character is more worthy of love than physical comeliness. From souls he ascends to the beauty of practices and laws, then to the beauty of different sciences and branches of knowledge.
At the end of the ascent, something unexpected happens. The philosopher does not deduce the Form of Beauty from the forms below it — he suddenly perceives it. Diotima uses the language of initiation and mystery: this is a vision, a revelation to which the whole preceding education was merely the preparation. What is seen is beauty absolute: not fair in one respect and foul in another, not beautiful to some and ugly to others, not dependent on any particular body or soul or institution — but beauty simple, separate, and everlasting, the cause of whatever beauty any other thing possesses.
Plato could have proposed an intellectual route to the Form of Beauty — a sequence of arguments or proofs. Instead he chooses eros. This is deliberate. The lover does not reason his way up the ladder; he is drawn up it by desire, which expands and refines as each lower object proves insufficient. The energy that begins in the longing for a beautiful body is the same energy that arrives, transformed, at the contemplation of beauty absolute. Philosophy, on this account, is not the extinction of erotic desire but its proper fulfilment — the desire for the beautiful, once purified, reaches its natural object.
The Ladder of Beauty is described in Diotima's speech in the Symposium, Chapter 6. Its influence runs through Neoplatonism and Renaissance humanism; Plotinus transformed it into a full metaphysics of ascent to the One.