Donald Smith wrote: "I guess you just have 'to do' something, and then try to justify it."
If a client improves after a therapist does something, what we have by way of "evidence" is a case of post hoc ergo propter hoc. But if we also have deterioration later after the therpist does something differently and improvement again when returning to the original intervention, we have N-of-one within-subject control comparisons.
My understadning of Control Mastery Theory is that, in the typical case, it does NOT justify the rationale for its interventions nor its theories apart from the presence of evidence of this general kind. This kind of evidence does not meet the research standards of rocket science, but when done as loyally as the researchers following Joseph Weiss's theories do it, this research is also not liable to the charge of being just ad hoc linguistic creativity. I concede that the change of mere linguistic creativity may however be applied with good reason to some, but not all, psychotherapeutic approaches.
I think there is no evidence that knowing all the major vairables influencing phenomena occuring in psychotherapy would necessarily be any more useful than knowing just a few major influences that account for most of the variance. Whether even a few variables are really known--in the sense of being known with certainty--is of course debatable and may be reasonably doubted. But making knowledge claims with certainty is also not the style of CMT theorists or therapists, as I understand their work.
And it is my understanding, furthermore, from research on the nature of human expertise, that human experts do not and probably cannot make good use of knowledge of more that a very few (about four or fewer) variables to predict or explain events associated with their interventions. This would suggest that a knowledge of all of the variables influencing important events in therapy may not improve anyone's skill in knowing what to do to implement interventions based on expertise. What a special knowledge of many more than a few variables (i.e., expertise) may do, however, that IS practical is guide experts in knowing how NOT to do interventions for a wide variety of situations.
Since Control Mastery Theory, in its respect for falsifying data, already has a built-in safeguard, against at least repetitions of ineffective interventions, the charge that they do not know all the variables affecting their interactions with clients further looses some of its force.
There is good reason to be greatly concerned that many psychotherapists have too little respect for the serious limitations of their knowledge claims. But inasmuch as psychotherapy is with us and we would find it hard to imagine society without it, I believe that the CMT folks may be highly commended for outstanding effort to bring a serious research program of high standard to their profession so that their knowlege claims have more credibility than they otherwise would have. Though the resulting effort is not rocket science, the difficulties that must be surmounted to carry out, as the CMT folks have, any kind of signficant process research that is faithful to the client's moment to moment changes in therapy, can be more daunting than those of rocket science. Far from lamenting the limitations of such a research program, I marvel with respect at the creativity and dedication of this group to finding sound empirical warrent for their work. That's the kind of research that makes possible the evolution of whatever knowledge may be possible about otherwise extremely elusive but real phenomena.