@hiya phil,
Well, Descartes' distinction between the substances of "thought" (mind) and "extension" (body) is his thesis. This thesis, though it looks superficially different in today's literature and overall acceptance, has not been wholly displaced.
One thing must be clear:
"the mind is part of the brain" (mereological principle)
and
"mental states are brain states" (identity thesis)
are not the same thesis. But even then, neuroscientists today
still attribute properties of the "mind" to the brain. So the "mind", though it has vanished explicit, its
properties are still alive and in employment.
Mental properties (in large part psychological, cogitative, cognitive, emotional, volitional powers/properties) are still being attributed in quite the same way Descartes attributed them. This is no different from Descartes error. Descartes need to explain the interaction between the mental and the physical; to simply subsume the mental
into the physical does not avoid the error because 'reducing' the problems to being a set of physical phenomena does not explain
why the mental is nevertheless different.
Different does not imply that the mental is some spooky realm of entelechies and spirits and minds. So there's no reason to suppose that the difference cannot be explain in some other terms; and it does not follow that simply assuming by fiat that the mental
just is the physical that you've
explained the mental. The identity thesis is highly controversial ground.
The problem is not
that the mind is different. The problem is that
what the mind is made up of is different. But what the mind is made up of is still being attributed to the brain (the identity thesis); or, neuroscientists find it
methodologically prudent to
putatively accept the identity thesis, along the lines of
functionalist interpretations and methodological orientation.