In clinical settings, the appeal to "the whole person" is most often an effort to remind us that the people we care for have dignity, agency and rights. We can easily think of situations in which the rights of people have been effaced in a bid to attack what were thought of as illnesses inside them; ice packs, insulin comas, lobotomies. All of these were attacks on a part of a person that failed to acknowledge their wholeness. This same effacement seems to have been part of what people were objecting to recently during the satire of the DefeatAutism Twitter hashtag. "By trying to defeat Autism", people protested, "you are trying to defeat me".
For some people, psychologist Bruce Hood is an example, the very idea of a self is an illusion. I'm not that compelled by Hood's thesis, which doesn't touch on the broader metaphysical questions about the experiences that arise out of the psychological processes he describes. In any case, in the early part of his book he points out himself that just because something is an illusion, it doesn't follow that it is not having a powerful real effect. So much for a putative lack of self, we still have something like "selfi-ness", and that is what we usually take to be important.
Not that kind of selfi-ness
The parts/wholes distinction is almost as old as clinical psychology (or possibly older), and as so often is the way, Paul Meehl has been there before. In his 1949 book Clinical vs. Statistical Prediction, Meehl anticipates the familiar argument that says that there is more to people than that which can be reduced to numbers:
“A cannon ball falling through the air is ‘more than’ the equation S= ½ g , but this has not prevented the development of a rather satisfactory science of mechanics”.
Meehl's point perhaps is that of course we should be interested in the "whole person", but that this does not preclude our being interested in measuring their parts. A caricature of quantitative psychotherapy research says that any interest in numbers or symptoms sees humans as "robots" or "microbes". This is no more the case for the psychotherapist than it is for the doctor who tries to impact someone's rich and complex life by monitoring simple changes in their temperature.
Undeniably there is something crucial to what we call "bedside manner"; that form of empathy and consideration that keeps the patient's experience in view even while working on the problem they present with. There is equally a power to seeing the impact of changes at the "parts"-level. Have you ever noticed how a simple change in something that bypassed your complex engagement with your self (something as simple as having a glass of water when you were thirsty) seemed nonetheless to enhance the way you felt at a more profound level? This is the direction that "symptom reduction" tries to go in, reduce the amount of time someone spends feeling bad and you can have immeasurable and intangible effects on their self as well.
People talk about psychotherapy research as though it should all have the descriptive quality of poetry or a novel. For sure, qualitative research and service user testimonials are an indispensable part of how we understand what is going on in the world, and when it comes to safeguarding people's dignity there is no substitute for individual narrative accounts. However, this approach (like all approaches) has its limits. The effort to "do justice" to people in all their complexity is not the only thing at stake in the effort to find out "what works" for them. Complicated creatures that we are, that complexity emerges from somewhere, and while we might feel like we are more than the sum of our parts, taking care of those parts can be surprisingly important.