In this delightful exchange between Wittgenstein and his fellow philosopher (and biographer) Elizabeth Anscombe, the legendary Austrian thinker uses the Copernican revolution to illustrate the point that how things seem is not necessarily how they are. Although it seems as though the sun goes around the Earth, it is of course the Earth rotating around its own axis that gives us night and day, and it is the sun, not the Earth, that sits at the centre of the solar system.
Nothing new here, you might think, and you’d be right. But Wittgenstein was driving at something deeper. His real message for Anscombe was that even with a greater understanding of how things actually are, at some level things still appear the same way they always did.
Every objects seem to have specific shapes and colours, and for the ones closer at hand, smells and textures too. This is how things seem. Although it may seem as though my senses provide transparent windows onto a mind-independent reality,and that perception is a process of ‘reading out’ sensory data, what’s really going on is – I believe – quite different.
Perceptions do not come from the bottom up or the outside in, they come primarily from the top down, or the inside out.
What we experience is built from the brain’s predictions, or ‘best guesses’, about the causes of sensory signals. As with the Copernican revolution, this top-down view of perception remains consistent with much of the existing evidence, leaving unchanged many aspects of how things seem, while at the same time changing everything.
This is by no means a wholly new idea. The first glimmers of a top-down theory of perception emerge in ancient Greece, with Plato’s Allegory of the Cave. Prisoners, chained and facing a blank wall all their lives, see only the play of shadows cast by objects passing by a fire behind them, and they give the shadows names, because for them the shadows are what is real.
The allegory is that our own conscious perceptions are just like these shadows, indirect reflections of hidden causes that we can never directly encounter. More than a thousand years later, but still a thousand years ago, the Arab scholar Ibn al Haytham wrote that perception, in the here and now, depends on processes of ‘judgement and inference’ rather than providing direct access to an objective reality.
Hundreds of years later again, Immanuel Kant realised that the chaos of unrestricted sensory data would always remain meaningless without being given structure by pre-existing conceptions, which for him included a priori frameworks like space and time.
Kant’s term noumenon refers to ‘things in themselves’ – Ding an sich – a mind-independent reality that will always be inaccessible to human perception, hidden behind a sensory veil. In neuroscience the story gets going with the German physicist and physiologist Hermann von Helmholtz. In the late nineteenth century, among a string of influential contributions, Helmholtz proposed the idea of perception as a process of ‘unconscious inference’.
The contents of perception, he argued, are not given by sensory signals themselves but have to be inferred by combining these signals with the brain’s expectations or beliefs about their causes.
In calling this process unconscious, Helmholtz understood that we are not aware of the mechanisms by which perceptual inferences happen, only of the results. Perceptual judgements – his ‘unconscious inferences’ – keep track of their causes in the world by continually and actively updating perceptual best guesses as new sensory data arrive.
Helmholtz saw himself as providing a scientific version of Kant’s insight that perception cannot allow us to know things in the world directly – that we can only infer that things are there, behind the sensory veil.
Helmholtz’s central idea of ‘perception as inference’ has been remarkably influential, taking on many different forms throughout the twentieth century. In the 1950s, the ‘new look’ movement in psychology emphasised how social and cultural factors could influence perception.
For example, one widely circulated study found that children from poor families overestimated the size of coins, while those from well-to-do families didn’t. Unfortunately, many experiments of this kind – while fascinating – were poorly done by today’s methodological standards, so the results can’t always be trusted.
In the 1970s, the psychologist Richard Gregory built on Helmholtz’s ideas in a different way, with his theory of perception as a kind of neural ‘hypothesis-testing’. According to Gregory, just as scientists test and update scientific hypotheses by obtaining data from experiments, the brain is continually formulating perceptual hypotheses about the way the world is – based on past experiences and other forms of stored information – and testing these hypotheses by acquiring data from the sensory organs. Perceptual content, for Gregory, is determined by the brain’s best-supported hypotheses.
After fading in and out of the spotlight over the half century since then, the idea of perception as inference has gained new momentum in the last decade or so. A variety of new theories have blossomed under the general headings of ‘predictive coding’ and ‘predictive processing’. Although these theories differ in their details, they share the common proposal that perception depends on brain-based inference of some kind.
The essential ingredients of the controlled hallucination view, as I think of it, are as follows. First, the brain is constantly making predictions about the causes of its sensory signals, predictions which cascade in a top-down direction through the brain’s perceptual hierarchies.
If you happen to be looking at a coffee cup, your visual cortex will be formulating predictions about the causes of the sensory signals that originate from this coffee cup.