Full Idea
One's life - not simply one's mental life - is centered in the activity of the simples that virtually compose one's brain in a way in which it is not centered in the activity of any of the other simples that compose one.
Clarification
'Virtually', here, means can be thought of in that way
Gist of Idea
One's mental and other life is centred on the brain, unlike any other part of the body
Source
Peter van Inwagen (Material Beings [1990], 15)
Book Reference
Inwagen,Peter van: 'Material Beings' [Cornell 1995], p.181
A Reaction
This justifies the common view that 'one follows one's brain'. I take that to mean that my brain embodies my essence. I would read 'centered on' as 'explains'.