When we see, hear, smell and touch, we’re not receiving reality—we’re creating it. Consciousness is a tapestry of predictions and fantasies, stitched together in the brain. A world outside of our heads may exist but is forever out of reach. These are radical conclusions of the world’s most influential and highly cited neuroscientist, Karl Friston. In this exclusive interview with the IAI he explains how his Free Energy Principle unifies physics and psychology, explains agency and dreaming, and promises new ways to understand and treat mental illness.

Karl Friston will be appearing on the debate The Structure of Consciousness at the HowTheLightGetsIn London 2025 festival this September 20-21, alongside speakers like Sir Roger Penrose, Sabine Hossenfelder, John Gray, Alastair Campbell, and many, many more. Book your place now.

Alasdair Craig: You say that conscious experience is a prediction generated by the brain. What does this mean?

Karl Friston: You could probably trace that back to the days of Plato, but to my mind it was beautifully articulated by the German physicist and physician Hermann von Helmholtz, in terms of the notion of unconscious inference. The basic idea is that the brain is a very constructive organ. It generates predictions, hypotheses, fantasies, explanations that best explain the sensory impressions on our eyes and our ears and our bodies. So this is very much, as people like Anil Seth and Andy Clark would say, an inside-out, a constructive, active process of generating predictions, as opposed to an outside-in process of passively absorbing data.

The idea is that if I’ve got a good grip on the world, if I’m doing good sense-making, then I should be able to predict exactly what is arriving on my sensory epithelia. In other words, if I’m tracking the world in the right kind of way, there will be no prediction error, there will be no mismatch between my predictions and what I’m actually sensing. And that leads to the notion of predictive coding, the imperative to minimize one’s prediction errors—the mismatch between what we predict should happen and the sensory evidence we receive. If the predictions are not correct then we have a prediction error and we have to update our beliefs.

Most of us probably assume that in perception you first get sensory input, and then you construct the world on that basis. But you’re saying it’s the other way round, you first construct a prediction, which is your world-model, then you get sensory input, and then you update your prediction.

You test that sensory input against your predictions. Literally, your brain cells can compare what’s coming in and what you thought should be coming in, and then the difference is the newsworthy information, it’s the bit you couldn’t predict. And then those prediction errors—the surprising sensory information—you use to update your beliefs, to drive changes in your brain, literally changing your mind, so that you adjust your model of the world to then generate better predictions.

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We are indeed trapped within our own heads, in many senses.

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Why is the brain doing it this way round? What problems does it solve by doing this?

The problem it solves is that we do not have access to the causes of our sensations. This is very often articulated in terms of the idea that the brain is skull-bound, it lives in the dark. It just has very sparse information through the sensory organs, sampling very small parts of the world.