Final answer:
Compatibilist free will and libertarian free will, as discussed by Peter van Inwagen, represent entirely different concepts within the philosophical discourse on determinism and moral responsibility.
Step-by-step explanation:
According to Peter van Inwagen, compatibilist free will and libertarian free will refer to entirely different concepts. This reflects option (a) from the provided choices. Compatibilism, sometimes referred to as 'Soft Determinism,' is the belief that free will can coexist with determinism, suggesting that freedom of choice is consistent with our actions being determined by prior causes.
In contrast, Libertarianism is an incompatibilist stance insisting that for free will to be genuine, our actions must not be determined by preceding events or causal chains, hence determinism must be false. Hard Determinists agree with Libertarians in rejecting compatibilism but arrive at a different conclusion, denying the existence of free will altogether. Critics of compatibilism, like Taylor, believe that it redefines the notion of freedom to make it compatible with determinism, making it appear as though there is some degree of choice when in fact all events are predetermined.