Developed by Carl Rogers in the 1940s and 50s, person-centred therapy was a radical break: the expert isn't the therapist, it's you. The premise is that people have a natural tendency towards growth, and it stalls when the conditions are missing — not when the techniques are.
Rogers named three conditions the therapist must provide: unconditional positive regard (you aren't evaluated, whatever you bring), empathy (the therapist actively tries to see the world from your position) and congruence (the therapist is a real person, not a professional role). His claim was that these three aren't preconditions for therapy — they are the therapy.
The idea became so influential that it's hard to see how radical it was. Nearly every modern school absorbed it; the "therapeutic alliance" invoked by CBT and psychodynamic work alike is Rogers's direct legacy.