Physical Determinism extends the laws of physics to the human mind.
If the laws of physics can explain the motions of the heavens as the result of natural laws, the same laws might explain human beings, including the individual mind.
Few scientists disagree with the view that all chemistry is applied physics, and that biophysics is completely consistent with the known laws of chemistry and physics.
There are emergent phenomena at the level of chemistry and many more in biology, but none of these require the old idea of "vitalism," phenomena inexplicable in physical terms.
When we come to the human mind, however, Descartes' old dualist ideas of an immaterial mental substance are alive and well. If not a separate substance, most "agent-causal" libertarians are looking for a currently inexplicable ability of the mind to initiate causal chains.
Neuroscientists resist these non-physical explanations. But they are only now beginning to see mental processes in near real time.