Epiphenomenon has two distinct meanings.

In the first sense, it is a side-effect or by-product, that is, there is a causal relationship between the secondary phenomenon and the first.

In the second sense, in medicine, such a relationship is not necessary. A doctor might ask a patient whom he is treating for a disease, "Have you noticed any new epiphenomena?"

In philosophy, the two senses are blended: epiphenomenalism is the doctrine that mental phenomena are secondary effects which cannot themselves cause anything.

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