Reductionism, it doesn't work, because sometimes the whole is bigger than the parts.

If one uses a reductionist approach to explain the relationship
between the 'mind' and the brain a reductionist approach
would not work because of 'emergence'. That is emergence of the
whole from the parts.
David Eagleman gives an example of the parts of an aircraft,
the individual metal hunks, screws and bolts, none of which
have the property of flight. Yet when assembled together we have
the property of flight.
The concept of emergent properties means that something new
may be introduced that is not inherent in any of the parts.