Van Inwagen famously argues for "incompatibilism"; viz., that the existence of free will is incompatible with "determinism." Van Inwagen writes: "Determinism is the thesis that the past and the laws of nature together determine, at every moment, a unique future (The denial of determinism is indeterminism)."Van Inwagen defines "the free will thesis" (not "free will") as follows: The free-will thesis is the thesis that we are sometimes in the following position with respect to a contemplated future act: we simultaneously have both the following abilities: the ability to perform that act and the ability to refrain from performing that act (This entails that we have been in the following position: for something we did do, we were at some point prior to our doing it able to refrain from doing it, able to not do it." (Ib., 329)
If the free-will thesis is not true then persons are not morally responsible for their actions.
Van Inwagen says that "the finest essay that has ever been written in defense of compatibilism—possibly the finest essay that has ever been written about any aspect of the free-will problem—, [is] David Lewis’s ‘‘Are We Free to Break the Laws?’’ (Van Inwagen - "Compatibilism is the thesis that determinism and the free-will thesis could both be true (And incompatibilism is the denial of compatibilism" - "How to Think About the Problem of Free Will," 330).
Van Inwagen is an example of how an analytic philosopher tackles the problem of free will. He is meticulous and rigorous. One must move slowly through him.
Here is a near-exhaustive collection of essays by Van Inwagen, to include his writings on the free will thesis.