It’s a testament to the strength and versatility of the human brain that anyone with at least half of one tends to assume that their senses give them direct access to objective reality. The truth is less straightforward and much more likely to induce existential crises: the senses do not actually provide the brain with a multifaceted description of the outside world. All that the brain has to work with are imperfect incoming electrical impulses announcing that things are happening. It is then the job of neurons to rapidly interpret these signals as well as they can, and suggest how to react.

This neurological system has done a pretty good job of modelling the world such that the ancestors of modern human beings avoided getting eaten by sabre-toothed tigers before procreating, but the human brain remains relatively easy to fool. Optical illusions, dreams, hallucinations, altered states of consciousness, and the placebo effect are just a handful of familiar cases where what the brain perceives does not correspond to whatever is actually occurring. The formation of a coherent model of the world often relies on imagined components. As it turns out, this pseudo-reality in one’s imagination can be so convincing that it can have unexpected effects on the physical body.