I sometimes see claims on the internet that according to epiphenominalism there is no free will. But what is the basis to this claim?
Epiphenomenalism is the view that mental events are caused by physical events in the brain, but have no effects upon any physical events. (SEP, Epiphenomenalism )
What I understand is that on this view the body seems to operate on its own and that consciousness or mind is a kind of byproduct of this operation.
If you consider this, then will is a mental attribute. If the mind is a by-product of the physical mechanism, then so is will (since it's a mental attribute).
The concept of free will seems to consider that one, having a mind, have "a significant kind of control over one’s actions" (SEP, Free Will)
But if one accepts that physical mechanism determines mental attributes, then Epiphenomenalists would have to conclude that there is no free will because it is determined by the physical mechanism as well.
This is again confirmed, when epiphenomenalism says mental events such as will has no effect on the physical world.
My thoughts There are chances to misunderstand this because we seem to have the power to exercise will in the physical world. An epiphenomenalist would probably say, well that is also a part of the physical process, as in, physical events causing physical events, only us the perceiving the event is the epiphenomenon.
Many people have argued that even materialism is compatible with free will (see, e.g., Can Free Will Exist In A Causal Material World?). If materialism is in fact compatible with free will, then it seems we can simply add the mental* events / qualia that epiphenomenalism adds on top of materialism, and still have compatibility with free will. Here, we need to distinguish "mental*" from "mental", where the latter refers to mental events in a physical sense. (Even epiphenomenalists will agree that there are some events that could be called mental that do have causal effects in the physical world -- neurons firing in the brain that make the body move, etc. -- but they argue there are also other mental* events -- qualia -- without causal influence.) Free will would then still fall under the mental rather than the mental*.
Of course one may argue that this is an awkward position to hold and that the will, if it is to be truly free, should be part of the mental*. But in my opinion, epiphenomenalism has bigger problems than that, including that one would be forced to hold that our talk about qualia isn't actually affected by any real qualia! (See here: How do epiphenomenalists make sense of discussions about qualia?)