The process of perception feels quite passive. We open our eyes and light floods in; the world is just there, waiting to be seen. But in reality there is an active element that we don’t notice. Our brains are always “filling in” our perceptual experience, supplementing incoming information with existing knowledge. For example, each of us has a spot at the back of our eye where there are no light receptors. We don’t see the resulting hole in our field of vision because our brains ignore it. The phenomenon we call “seeing” is the result of a continuously updated model in your mind, made up partly of incoming sensory information, but partly of pre-existing expectations. This is what is meant by the counterintuitive slogan of contemporary cognitive science: “perception is a controlled hallucination”.
A century ago, someone with an interest in psychology might have turned to the work of Freud for an overarching vision of how the mind works. To the extent there is a psychological theory even remotely as significant today, it is the “predictive processing” hypothesis. The brain is a prediction machine and our perceptual experiences consist of our prior experiences as well as new data. Daniel Yon’s A Trick of the Mind is just the latest popularisation of these ideas, but he makes an excellent guide, both as a scientist working at the leading edge of this field and as a writer of great clarity. Your brain is a “skull bound scientist”, he proposes, forming hypotheses about the world and collecting data to test them.
