8.1 The Helmholtz move

In 1867 — eight years after Darwin and twelve after the discovery of the speed of light — Hermann von Helmholtz wrote a passage in his Handbook of Physiological Optics that has shaped every conversation about perception since:

“It may often be quite difficult to say how much of our apperceptions… is due to immediate sensation, and how much, on the other hand, is due to experiences and training. The general rule determining the ideas of vision that are formed whenever an impression is made on the eye, with or without the aid of optical instruments, is that such objects are always imagined as being present in the field of vision as would have to be there in order to produce the same impression on the nervous mechanism.”

In other words: we perceive what would have caused the sensation we are receiving, based on prior experience. Helmholtz called this unconscious inference, and it is the seed of every modern theory of perception, including predictive coding, Bayesian-brain models, and active inference.

The picture: at any moment, the brain has a representation of the causes of its sensations — the objects, events, and configurations in the world that would produce the current sensory input. Perception is the process of finding the most likely causes given what is being sensed. It is, mathematically, an inference problem.