Epiphenomenalism is the belief that our consciousness has no causal effect on the world. This is an attitude most often expressed by dualists, who, in accepting that the material world is causally closed, are forced to this conclusion. It is also possible to arrive at epiphenomenalism through a purely materialist position, if one concludes that consciousness is simply a residue or by-product of the operation of the brain (for example a certain reading of biological naturalism could lead to this conclusion).
Do we have reason to believe that this thesis is likely, or at least possible? At first it seems that we have an obvious rebuttal: if our consciousness doesn’t have a real causal effect then how can we explain our experience? Surely it seems like our will or desire to do something has an effect on the world. It would be astronomically improbable to suppose that our minds and the physical world simply happened to align by coincidence, especially if you aren’t willing to claim that there is some super-natural causality-violating force such as god to ensure the proper alignment.
Epiphenomenalism does have a possible response to this objection. If we return to the hypothesis that consciousness is a causally ineffective residue of the brain then it is possible that our desire to act in a certain way is created by the brain’s physical processes, the very same processes that caused the action to occur. Our previous argument against epiphenomenalism assumed that consciousness and the brain were running on basically their own courses, so let us now assume instead that the conscious state of a person at a given moment is a by-product solely of their physical state at that moment, thus eliminating the possibility of mismatches. (It is also a corollary of epiphenomenalism and the above statement that consciousness has no real causal powers at all, not even on subsequent conscious states.)
Digging into that claim reveals some interesting consequences. First is that alterations to a brain state have an effect on the mental state that is produced by it. The second consequence is that the mental state is determined completely by the physical states of the brain. If it wasn’t there would be some cases in which the mental state mismatches with the physical, which as we know doesn’t happen.* However, those two consequences are enough to show that mental states and brain states are dependentical, as argued previously. And once you have shown that something is a description or abstraction of the physical you can also show that it has causal powers, as argued here.
Thus no matter how you interpret it epiphenomenalism a bad theory about consciousness, as it is either simply too improbable to believe or it collapses into some kind of identity theory (although not necessarily the theory that is traditionally referred to as “identity theory”, functionalism for example can be seen as a kind of identity theory).
* I know that some of you may object, claiming that times when we are immobilized by drugs are exactly these kinds of mismatches. However, the parts of the brain that are responsible for causing your conscious desire to perform the action are not actually affected, only the last messengers, and hence there is no mismatch between the part of the brain “causing” the will to act and the conscious realization of that will.