What is Freud's single greatest contribution?

Freud already answered this question, of course, when he said that whereas Darwin decentered the human animal from the apex of creation, Copernicus decentered the earth from the center of the universe, Freud had decentered humanity in itself. No longer was the human immanent.

Lacan’s answer is perhaps even better: “That is why the unconscious was invented—so that we would realize that man’s desire is the Other’s desire, and that love, while it is a passion that involves ignorance of desire, nevertheless leaves desire its whole import”

Credit to Alex Colston.

A short intro can be found here, Jacques Lacan. He was born in 1901 and referred to as the French Freud. His most famous dictum is “the unconscious is structured like a language.” The above has another one, “man’s desire is the desire of the Other.”

It reminds me another quote from Marcel Proust, “We fall in love for a smile, a look, a shoulder. That is enough; then, in the long hours of hope or sorrow, we fabricate a person, we compose a character.”