Consider concepts like soul or consciousness: does a person have a soul? are they conscious. One possibility, which those ardent supporters of evidence-based reasoning will outright reject due to the way it inconveniences their cherished 'evidence-based' beliefs is that:
- Some people are conscious, others are not; and yet
- Given only material evidence, we cannot tell them apart.
Obviously by construction, if this is the case, then an evidence-based approach will fail to discern between two people, one conscious, one not, who present similarly on the Exterior. Yet how can one rule such a possibility out without effectively begging the question by assuming such possibilities away.
With the impenetrability and complexity of mind and brain we have something quite distinct from physical medicine. To use a simple physical analogue, physical medicine (that is, medicine where the mind and brain process may be neglected), is like a simple pendulum, its behaviour easy to predict based on easily measurable properties of the pendulum, such as if you measure the length reasonably accurately, you'll get a reasonably accurate model of its behaviour; whereas psychiatric medicine, when the brain can no longer be neglected, and where the brain's behaviour may no longer be simplified by taking assumptions which may be reasonably expected to hold in physical medicine, given complete knowledge of the parameters of the double pendulum to the same reasonable level of accuracy as we did with the single pendulum, we have no capacity to make accurate long-term predictions as to the double pendulum's behaviour.
In the face of brain complexity, the naive paradigm of an evidence-based paradigm like evidence-based-medicine will fail.