
In Lewis Carrol's famous book,
Alice's Adventures in Wonderland (1865), a race is suggested by the Dodo bird after various characters get wet from Alice's tears. The race is conducted in an irrational way with no fixed starting point or finishing line but the Dodo is called upon to judge who had won and who would get the winner's prize. His adjudication was that 'everyone has won and all must have prizes'.

This famous Dodo bird verdict was applied to the field of psychotherapy in 1936 by Saul Rosenzweig who believed that all psychotherapies worked about the same because of shared common factors. Rosenzweig
died in 2004 at 97 still committed to teaching even though he had formally retired at 1975.
His conclusions have been supported by a number of researchers (Frank; Luborsky et al.; Weinberger) but strongly affirmed by Duncan (2002) who
interviewed Rosenzweig at 93 having believed him to have died! Duncan sorted the common factors into the categories of client, relationship, placebo and technique believing that the contribution that each of these makes to psychotherapy's effectiveness is 40%, 30%, 15% and 15% respectively. Duncan opined that psychotherapy ought to take much more notice of the qualities and potential of clients to play a large role in their healing rather than be fixated on client techniques.
However, the common factor tradition has been strongly opposed by those who believe that research should continue to identify specific therapies for specific disorders under specific conditions. Such endeavour is the hallmark of so-called empirically-supported therapies (EST).
It seems to me that these two traditions will never quite meet because the latter is wedded to the idea that scientific research is the only pathway to truth; the earlier, common factors movement, while it esteems science also believes that the scientific study of technique alone cannot sum up all that psychotherapy is.
My sympathies lie with the common factors approach while allowing that certain, specific conditions (eg phobias) may be better helped by a specific therapy (eg behaviour therapy).