In Hasker’s book, Metaphysics, he discusses two different views regarding freedom: Determinism and Libertarian Free Will. In assessing Determinism, he gives the following argument which he believes undermines any rationality if one holds to that worldview:
- Every thought or belief accepted by a person is a result of that person’s brain being in a corresponding state.
- We assume, provisionally, that the physical indeterminacy which exists at the quantum level makes no perceptible difference in the overall functioning of the brain. So that the brain functions, in effect, as a deterministic system.
- Therefore every brain state, and therefore every thought and belief of the person, is fully determined by the physical functioning of the brain in accordance with the deterministic laws of physics. 
Hasker also states that one makes decisions about what to believe based on evidence and evaluating claims to see if they are justified by evidence. He calls this “rational insight”, which is guided by the principles of sound reasoning. To be clear, determinism is defined as, “for every event which happens, there are previous events and circumstances which are its sufficient conditions or causes, so that, given those previous events and circumstances, it is impossible that the event should not occur.” With this definition in mind, let’s move forward in explaining the following premises.
It seems from the definition of determinism which Hasker gives, that premise 1 fits in well with the boundaries given for understanding determinism. If one does accept a belief, it is not because they actually chose to, but because of prior states of affairs, they could have not believed, or believed in the present proposition. The determinist would interpret the idea of rationally choosing a proposition to believe in, to be illusory, no matter how dramatic the choice may be. To solidify my evaluation, consider the following example. Bob, an accountant, is in his office working with some clients when one of them says, “The Vegas Knights will definitely win the Stanley Cup!” At that moment, Bob has thoughts about the statistics he read on the probability of the knights winning the Cup, as well as the average points per team and how they compare. Bob seemingly weighs the evidence, (which is actually just certain neutrons firing) and concludes that the Knights will not win the Cup. But, even though it seems to Bob that he did indeed make a decision, he actually did nothing of the sort. He was causally determined to think that the Knights would not win regardless of how strongly it seems he evaluated evidence and came to a decision.
Because I have almost no knowledge of QM, I can’t really state any points for or against premise 2, but essentially it just seems as though Hasker is conceding that QM doesn’t actually threaten determinism (for sake of argument) in order to make his point against determinism that much more potent.
This premise also seems self-explanatory, except that there is an assumption being made on Hasker’s part in regards to the question of consciousness. It seems he assumes that the determinist will in fact hold to a physicalist view of the mind (that the brain and mind are identical) as he states that thoughts and beliefs are the same substance as brain states. Or, at the very least, that brain states causally determine our thoughts and beliefs (again, because of my unfamiliarity with the mind-body problem, I can’t really comment on this in a reasonable manner). So, it seems that Hasker’s argument will succeed only with deterministic views that suppose a) that the brain and the mind are identical or b) that the brain causally determines our thoughts and beliefs.
Overall, Hasker’s premises seem plausible under a deterministic framework, but some possible objections do arise to Hasker’s conclusion.
Objection: Rationality is a Causal Factor in regards to beliefs
One of the first objections which come to mind is positing the idea that rationality is actually a causal determiner of our thoughts and beliefs, as it is a law which acts upon our brains like any other law. To provide evidence of this, it seems reasonable that because the universe in which we live is governed by laws which are predictable (that is to say, they are uniform in effect) so the law of gravity as well the law of identity or the law of excluded middle all are laws which affect us. So, is it really irrational to think that there is a law of rationality which governs our thinking so that we can still be rational without actual choice in our intellectual pursuits?
Counter Objection: No Empirical Evidence
One test of a theory is to examine whether or not it is factually correct, or aligns with what one sees in reality. Assuming rationality is a ‘law’ of nature, what would we expect to see? For one, it is natural to think that there would be no conflict in deciding what is true or false in society. If rationality is indeed a law of nature, then there would be no other possible choice between one argument and another! There would no longer be any falseness in how we understand arguments, there would only be truth. Obviously this must be false.
This counter objection may fall prey to the Straw Man Fallacy, in that it assumes that the determinist will deny that there is no choice when it comes to determining truth, which isn’t true. The determinist will ultimately deny there is no choice, but that does not mean the illusion of choice is not present. Just as with other choices in life that the determinist will say are causally restrained to a single option, that does not mean there is no apparent option to the agent, but the final interpretation of that decision is that it is apparent and not real. In the same way, the determinist will posit that one can still feel they have a choice between a proposition being true or false, but ultimately there is no choice. Thus, the counter objection may be a straw man of the determinist view; but is that the case?
Upon further reflection it seems the accusation of strawmanning the determinist view is a false one. To see this, one needs to consider the difference between an agent’s choice being causally determined through sufficient conditions, and positing a law which determines how one comes to understand the world around them. The former leaves room for an interpretation of the data of apparent choice, whereas the latter adjusts the data itself by inserting a law which epistemically constrains mankind to one possible choice in which is inescapable as the determinist who holds that rationality is a law will only believe in that which is true. Disagreement is impossible.
While Hasker’s argument may be short, it is quite substantive. In exposing the deficiencies in the determinist worldview in regards to epistemology, it undermines anyone’s claim to truth via rational insight under a determinist framework. While there may be objections to his idea, they seem to fall short and thus if one is not content to be a epistemic nihilist, rejecting determinism must follow.
 Hasker, Metaphysics, 47-48.
 Ibid., 47.
 Ibid., 32.
Hasker, William. Metaphysics: Constructing a World View. Downer’s Grove, IL: InterVarsity Press, 1983.