If a tree falls in a forest and there’s no one there to see it, does it make a sound? An age-old philosophical conundrum you might think; in fact, this question was given a definitive answer in the 17th century by the father of modern science, Galileo Galilei. And the way in which Galileo answered this question shaped the philosophical foundations of the scientific worldview that remains with us to this day. Moreover, as I will explain, this scientific worldview has a big problem at its heart: it makes a science of consciousness impossible.

A key moment in the scientific revolution was Galileo’s declaration that mathematics was to be the language of the new science; the new science was to have a purely quantitative vocabulary. This is a much-discussed moment. What is less discussed is the philosophical work Galileo had to do to get to this position. Before Galileo, people thought the physical world was filled with qualities: there were colors on the surfaces of objects, tastes in food, smells floating through the air. The trouble is that you can’t capture these kinds of qualities in the purely quantitative vocabulary of mathematics. You can’t capture the spicy taste of paprika, for example, in an equation.

This presented a challenge for Galileo’s aspiration to exhaustively describe the physical world in mathematics. Galileo’s solution was to propose a radically new philosophical theory of reality. According to this theory, the qualities aren’t really out there in the world, rather they’re in the consciousness of the observer. The redness of the tomato isn’t really on the surface of the tomato but is rather in the consciousness of the person perceiving it; the spiciness of the paprika isn’t really in the paprika but in the consciousness of the person consuming it. To return to the example we began with, when a tree comes crashing down in a forest, the crashing sound isn’t really in the forest, but in the consciousness of an onlooker. No onlooker, no consciousness, no sound.

Galileo, as it were, stripped the physical world of its qualities; and after he’d done that, all that remained were the purely quantitative properties of matter—size, shape, location, motion—properties that can be captured in mathematical geometry. In Galileo’s worldview, there is a radical division between the following two things:

  • The physical world with its purely quantitative properties, which is the domain of science,
  • Consciousness, with its qualities, which is outside of the domain of science.

It was this fundamental division that allowed for the possibility of mathematical physics: once the qualities had been removed, all that remained of the physical world could be captured in mathematics. And hence, natural science, for Galileo, was never intended to give us a complete description of reality. The whole project was premised on setting qualitative consciousness outside of the domain of science.

What do these 17th century discussions have to do with the contemporary science of consciousness? It is now broadly agreed that consciousness poses a very serious challenge for contemporary science. Despite rapid progress in our understanding of the brain, we still have no explanation of how complex electrochemical signaling could give rise to a subjective inner world of colors, sounds, smells and tastes.

Although this problem is taken very seriously, many assume that the way to deal with this challenge is simply to continue with our standard methods for investigating the brain. The great success of physical science in explaining more and more of our universe ought to give us confidence, it is thought, that physical science will one day crack the puzzle of consciousness.

This common approach is, in my view, rooted in a profound misunderstanding of the history of science. We rightly celebrate the success of physical science, but it has been successful precisely because it was designed, by Galileo, to exclude consciousness. If Galileo were to time travel to the present day and hear about this problem of explaining consciousness in the terms of physical science, he’d say “Of course you can’t do that! I designed physical science to deal with quantities, not qualities.” And the fact that physical science has done incredibly well when it excludes consciousness gives us no grounds for thinking it will do just as well when it turns to explaining consciousness itself.

This is not to say that physical science has no role to play in the science of consciousness. Neuroscientists have made great progress in mapping correlations between brain activity on the one hand and conscious experience on the other. Giulio Tononi’s integrated information theory of consciousness, to take a prominent example, proposes that consciousness is correlated with maximal integrated information, a notion for which the theory gives a precise mathematical characterization. But mere correlations are not a theory of consciousness.

What we ultimately want is a way of explaining these correlations that neuroscientists uncover. Why is it that maximal integrated information, a quantitative property, always goes along with consciousness, a qualitative phenomenon? The problem is that our adoption of Galileo’s view of the physical world blocks us from answering this question. Consciousness is essentially defined by the qualities—colors, sounds, smells, tastes—that characterize every second of waking life.

And those qualities, by definition, cannot be incorporated in a purely quantitative picture of the physical world. The Galilean understanding of the physical world as purely quantitative bars us from bringing together the qualitative and the quantitative in a single, unified picture of reality. The best we can do is to map correlations.

Pessimists will infer from these considerations that we will never have a science of consciousness, that consciousness will always be something magical and mysterious. That’s not my approach. I think we can have confidence that we will one day have a science of consciousness, but we need to rethink what science is. The science of Galileo was not designed to deal with consciousness. If we now want a science of consciousness, we need to move to a more expansive "post-Galilean" conception of the scientific method, one that takes seriously both the quantitative properties of matter than we know about through observation and experiment, and the qualitative reality of consciousness that each of us knows through our immediate awareness of our feelings and experiences.

Nothing short of a revolution is called for, and it’s already on its way. As I describe in my new book Galileo’s Error: Foundations for a New Science of Consciousness, scientists and philosophers have begun to come together to lay the groundwork for a new approach to consciousness. And this matters. The change in worldview that is called for cannot help but have profound implications for society more generally. Consciousness is at the root of human identity; indeed, it is arguably the basis of everything of value in human existence. This new scientific revolution will transform not only our understanding of the physical universe, but also of what it means to be a human being.