How did Thomas Henry Huxley contribute to epiphenomenalism?

In the context of the philosophy of mind, the biologist Thomas Henry Huxley is recognized as the best-known modern proponent of epiphenomenalism.2

Epiphenomenalism is a theory suggesting that mental states—such as thoughts, desires, or sensations—are by-products of brain activity. According to this view, while these mental events are necessarily caused by physical states in the brain, they possess no causal power of their own; they do not influence the brain or the body.12

Huxley’s contribution lies in his articulation of this mechanistic perspective, where the mind is viewed as an ineffective "shadow" of physical processes. In this framework, when a person wills a bodily movement, the movement is actually caused entirely by prior physical states of the body, rather than by the mental act of will itself. Consequently, while the act of will seems to the subject to be the cause of the movement, it is merely a secondary effect of the underlying physical events.2