Some feel that if one truly embraces a materialist view concerning the world, meaning that there is no such thing as a mental substance, and that all is physical, then it becomes nonsensical to talk about moral responsibility, as it is really the physics of the situation and not the person who is responsible. Before I discuss why this is a false conclusion I must make clear some of the assumptions the argument I am presenting will be making. First I am making the assumption that materialism or physicalism necessarily implies determinism, at least with respect to free will. Some might object that quantum physics invalidates this assumption, but realistically quantum uncertainty does not make you freer than you were before. It seems unlikely that you have any influence over how quantum states in your brain collapse, since you do not directly observe the contents of your brain in any fashion that would cause them to collapse (how active is neuron #296 in your parietal lobe at the moment?). Even worse (for free will) if the multiple worlds interpretation of quantum mechanics is correct than you are constantly taking all possible courses of action, moral and immoral. My other assumption is that we are considering the impact of determinism with respect to a moral theory that holds that goodness is an objective fact, not an artifact of our language. There are coherent views which propose that our use of “good” is simply a linguistic device, used to influence the actions of other people, and such a view would not be affected by determinism, while a view holding that the good was an objective fact would, since such views usually hold that one can only be moral or immoral where one has a choice in one’s actions.
First let me show why it is still rational to punish people for their actions in a deterministic universe. The goal of punishment is to prevent the wrongdoer from making their particular mistake again, as well as to make an example that will dissuade others from doing wrong. Both of these goals are still obtainable under determinism. For example even if the wrongdoer’s future actions are completely determined by the current state of the world it is still reasonable to assume that their future actions would be different in a world that includes them being punished versus a world that did not include their punishment. (In a quantum mechanical view the probability distribution of their future actions would be affected.) Likewise the inhabitants of a world that includes punishment will behave differently than a world that does not. The key idea here is that determinism doesn’t imply that a person’s actions are fixed by themselves but are fixed with respect to the state of the entire universe; there is still cause and effect in a deterministic universe.
As you would expect moral responsibility is harder to deal with than the practical issue of punishment. We wonder if it makes sense to simply blame the world for one’s actions (“physics made me do it”). However the natural laws cannot be the cause of an action, although they determine how interactions between particles proceed they do not actually interact with the particles. If an electron is bumped out of its orbit by a photon the cause is the photon, not natural law. Thus the cause of a person’s actions is the biological processes in their brain, not natural laws. Of course these biological processes themselves have causes, namely their earlier states as well as earlier input from the outside world. The primary cause for any state in the biological system in question however is usually an earlier state of that very biological system. In more conventional terms we would say the primary cause of a brain state is earlier brain states. However since we are accepting materialism as one of the premises of this argument this is the same as saying that the person is the cause of their actions. What about those earlier inputs? Well they do contribute somewhat the person’s actions, so perhaps we should say that the person is mainly to blame, although their school teachers (for example) share some of the responsibility because they did not instruct them correctly. This description of the cause of a person’s actions is the same as that which we get when we analyze them under a free will theory, which means that our notions of moral responsibility are not affected by determinism.