By the nature of the following refutation of determinism, it can also be considered a proof (1) of (libertarian-type) volition. It is a refutation of any theory that would deny the existence of non-necessitated events–in the form of fundamental, agent-controlled choices–located in a conceptual consciousness, including incompatibilist determinism and compatibilism.
Here is a short video (by someone else) explaining the general outlines of the argument:
Here is a link to the paper by Harry Binswanger:
(1) The argument shows that volition is a prerequisite of all knowledge and that the denial of volitional choice implies self-contradiction, thus establishing volitional choice as an axiom of epistemology. Strictly speaking, I think, by Leonard Peikoff’s definitions, this is not a proof, but an axiomatic validation. It establishes the axiom as absolutely and unquestionably true, but does not involve the specific process of logical proof of the positive proposition. The positive proposition is self-evident in each individual’s experience of existence.