Imagine you and I are walking together along Brighton seafront on a day bathed in sunshine, and we both stop to gaze up at the deep blue sky. It’s a beautiful sight, but are we having the same experience? Do you see the same blue that I see? It’s easy to assume that we do. After all, we both use the word “blue”, and the colour seems to be a property of the sky, not of our minds. But the science of perception – of how the brain interprets sensory information to bring forth objects, people and places – suggests otherwise. Just as we all differ on the outside, it’s likely that our inner experiences differ too. It may seem as though the world just pours itself directly into our minds through the transparent windows of our eyes and our ears. But psychologists have long known that perception is not simply a “read out” of sensory information. We are strongly influenced by context. From the effect of shadows on how we perceive the brightness of a surface, to our tendency to interpret facial expressions depending on what we think is happening, context permeates all our conscious experiences, and it does so in a way that we are typically never aware of. Some researchers, myself included, go even further. Instead of context merely influencing the contents of perception, the idea here – which builds on the legacy of the great German polymath Hermann von Helmholtz – is that perceptual experience is built from the top down, with the incoming (bottom-up) sensory signals mostly fine-tuning the brain’s “best guesses” of what’s out there. In this view, the brain is continually making predictions about the causes of the sensory information it receives, and it uses that information to update its predictions. In other words, we live in a “controlled hallucination” that remains tied to reality by a dance of prediction and correction, but which is never identical to that reality. A striking consequence of this is that since we all have different brains, making slightly different best guesses, we will all have different perceptual experiences too – even if we are faced with the same objective external reality. Just as the blueness of the sky may be different for each of us, all our experiences may differ – does a peach taste the same to me as to you? Unlike our external differences, differences in perception are private, subjective – hidden beneath the common language we use to describe them. Well, not always. Some kinds of perceptual diversity are very familiar. Descriptions of hallucinations and delusions go back thousands of years, but they are usually interpreted as a failure of perception against the benchmark of an idealised, normative way of perceiving the world. More recently, the concept of “neurodiversity” has called deserved attention to the radically different ways some people experience their worlds. This framework emphasises that those differences are not deficits, though this emphasis is often lost because neurodiversity is typically associated with medically defined conditions such as autism or ADHD, which are usually thought to be difficult to manage. (There are some exceptions. Synaesthesia – often described as a “mixing of the senses”, where you might taste colours or see sounds – is often seen as enhancing creativity and cognitive ability, though the evidence is mixed.) What is missed by the idea of neurodiversity is the possibility that every one of us sees the world in our own way, though not so markedly that the differences surface in how we behave or describe our experiences. Sometimes the spell is broken and we recognise our perceptions for the constructions that they are. A few years ago, a poorly exposed photograph of a dress tore across social media because half the world saw it one way (white and gold) while the other half saw it another (blue and black). People in each camp just could not believe that the other interpretation was possible. This might have been a lot of fun, but it didn’t seem to lead to a broader understanding that the way we see (or hear, smell, taste or touch) things might differ from person to person. A new project I’m involved with, called the Perception Census, aims to change all this. Developed by a team that includes scientists, philosophers and artists, the goal is to map out the hidden landscape of perceptual diversity. It consists of engaging, fun, easy and quick-to-complete online experiments and interactive illusions. As well as contributing valuable data, participants can learn about their own powers of perception and how they relate to others. Importantly, the Perception Census goes beyond visual perception, exploring sound and music, emotions and how we experience the passing of time. Anyone over 18 can take part and the first results will be released by the end of the year. Bringing to light our inner diversity could be as transformational for society as recognition of our externally visible diversity has been. Just as there is no “best” skin colour, there is no single way of perceiving the world against which others can be compared and found wanting. And unlike the concept of neurodiversity, which tends to be reserved for specific conditions, perceptual diversity applies to all of us. A greater appreciation of this should help us cultivate humility about our own perspectives. Just as it serves us well to occasionally question our social and political beliefs, it’s useful to know that others can literally see things differently to us, and that these differences may evolve into different beliefs and behaviours. It could even be that engaging people in simple exercises that reveal the constructed nature of perception will open their minds to other viewpoints and help resolve disagreements. Mostly, though, an understanding of perceptual diversity just makes the world a richer, more wonderful place. Next time you go for a walk by the sea, or through a town, try to imagine all the unique, personalised inner universes in the minds of all the people around you. There isn’t just one beautiful world out there, there are many. Anil Seth is professor of neuroscience at the University of Sussex and the author of Being You – A New Science of Consciousness (Faber). Further reading Other Minds: The Octopus and the Evolution of Intelligent Life by Peter Godfrey-Smith (HarperCollins, £9.99) The Brain – 10 Things You Should Know by Sophie Scott (Orion, £9.99) Wednesday Is Indigo Blue: Discovering the Brain of Synesthesia by Richard E Cytowic and David M. Eagleman (MIT, £30)