Whilst Cartesian dualism argues that there is a two-way interaction between mental and physical substances, not all forms of dualism agree. Epiphenomenalism argues that mental events are caused by - or are a by-product of - physical events, but that the interaction is one-way: mental events cannot affect physical ones. The analogy often used is that of the smoke that comes from a factory which is a by-product of its running, but does not actually affect its running.

One of the curious side effects of this theory is that it implies that decision making is not a mental event. Apart from flying in the face of most common sense attitudes, this consequence throws up a further problem: how are decisions made? Is decision making a bodily process? If so, what difference is there between epiphenomenalism and certain forms of materialism?

Epiphenomenalism is often confused with materialism but this is in fact a misunderstanding. The reason for this is the way in which the theory classes mental events as secondary, leading to the view that physical events are primary. While this may be true, what most people miss is the fact that mental events are said to be caused by physical events. This being so, mental events cannot be identical with events, just as smoke cannot be identical with the fire which causes it.