TOK essay (Christopher Hitchens's quote) do you agree?

The fact that mental activity or rumination is entirely determined by the physical processes of the brain.

Yeah... no.

Aside from the classical Platonic stance and the branches of Cartesian Dualism that contest this there are also schools of Emergent Materialism (see Sean Sayers et al.) and Extended Cognitive Functionalism (Wheeler et al. ) which have developed in response to this problem. The claim that all mind activity is determined only by material phenomena is - to put it mildy - hotly contested. That approach is unable to provide an adequate conceptual model of cognition on any level other than "what can we measure hurr durr", which is a fairly low level approach to take to the problem.

Not be any respectable scientist it isn't.

The number of potential neuronal circuits in the brain greatly exceeds even the number of atoms in the universe, and you seem to think there a need for non-material activities of the mind also? Please.

It's pretty good at explaining brain function better than anything antecedent to it, and it is pretty good at recognising that when parts of the brain are physically damaged, it completely changes the functioning of the mind.