I disagree. Psychology is based on subjective interpretation of subjective observations of the outward expressions of others filtered through the sieve of the subjective interpretations of prior work.
Psychology and neurology go hand in hand, and data backs each other up (and data from one side often spurs research by, or answers questions of the other)
Psychology is subjective, but there is a wealth of hard facts in prior neurological research that points to very specific things.
Here's the kicker: without thousands and thousands of dollars for brain scans (and/or neurotransmitter tests) for each individual patient, it is insanely difficult to always find the root physiological cause of any neurotransmitter imbalance (THE ultimate cause of any "psychological" issue). It's often highly informed guesswork, be it by a medical or psychological practitioner.
Like depression: it could be temporary, caused by nutritional/lifestyle/environmental issues; it could be semi-permanent, caused by a host of disorders in the body (such as a "malfunctioning" organ or autoimmune or what have you); it could also be a permanent "wiring" issue in the CNS itself.