What kind of scientist thinks you can't study behavior or mental health scientifically? The opportunities to do so reductively might be somewhat limited, but that's no different from a lot of medicine. You have some successes (tamoxifen works; venlafaxine works) and some failures (cabozantinib doesn't work; aprepitant doesn't work), both in mental health and in other complex situations like cancer. (I gave cancer drug / mental health drug pairs above.)

Anyway, no, we absolutely don't have to assume the mind isn't relevant. Good grief. What a silly idea! What do you think cognitive behavioral therapy is targeting? When it shows efficacy, what is it, do you think, that people are measuring?