Vienna 1902 – London 1994
Karl Popper
There are three worlds: physical things, conscious minds, and the products of minds, such as theories, problems and arguments. The products are real, because they change the physical world, and they can only do so through minds. So minds must be real, and they must make a difference.
The view
Popper is best known for his philosophy of science. Theories can never be proved true, only tested, and a theory is scientific if it says what would show it false. No number of white swans proves that all swans are white, but one black swan refutes it. Knowledge grows by bold conjecture and severe criticism. That philosophy, critical rationalism, is the starting point of David Deutsch's work and shapes the thinking of many scientists.
Less well known is that Popper held a strong view about the mind. In Objective Knowledge (1972) he divided reality into three "worlds". World 1 is physical things and processes. World 2 is mental states: perceptions, feelings, thoughts, intentions. World 3 is the objective products of the human mind: theories, problems, arguments, works of art, and the logical relations between them. In The Self and Its Brain (1977), written with the Nobel-winning neurophysiologist John Eccles, he defended interactionism: the mind is real, it is not identical with the brain, and the two act on each other.
The argument
World 3 changes World 1, and it can only do so through World 2. After Popper, The Self and Its Brain (1977)
Popper's main argument runs through World 3. Theories are not physical objects: a theory is not the ink in a book or the pattern in a brain, since the same theory can be written in many books, and it has consequences nobody has yet noticed. Yet theories plainly change the physical world. The theory of nuclear fission led to power stations and bombs. For a theory to have physical effects, someone must grasp it, reason with it and act on it. That grasping is a mental act, a World 2 event. So World 2 must be real and causally effective. Here Popper sides firmly against Huxley's steam whistle.
He also argued that physicalism undermines itself. If our beliefs were fixed entirely by physical causes, with no room for being moved by reasons, then our acceptance of physicalism would itself be the product of causes, not of argument. And he drew on the psychologist Karl Bühler's account of the functions of language. Animals and machines can express states and send signals, he said, but describing the world truly or falsely, and arguing validly or invalidly, are higher functions that physical theories have not explained.
Popper's most fruitful idea about minds, though, is evolutionary. Better to let our hypotheses die in our stead, he liked to say. A creature that can try out actions in imagination, and discard the bad ones before acting, has a huge advantage over one that must try them in the world and live with the results. Run the figure below.
Where he meets Dennett
On evolution and the growth of knowledge, a long way. Dennett built Popper's idea into his "Tower of Generate-and-Test" in Darwin's Dangerous Idea (1995) and Kinds of Minds (1996). Darwinian creatures are designed by natural selection alone. Skinnerian creatures learn by trial and error. Popperian creatures, named in Popper's honour, can test possible actions against an inner model of the world before they act. Gregorian creatures, the next floor up, use tools and culture, and above all language, to think better still. Both men saw minds as products of evolution, and knowledge as something that grows by variation and selection rather than by induction from data.
They also share a dislike of philosophy that hides in the meanings of words. Popper's war on "essentialism" and Dennett's war on armchair intuition are cousins.
Where he parts
On the mind itself, they are far apart, which is why Popper sits towards the far end of the map. Popper's interactionism is close to the dualism Dennett spent his career opposing. Dennett would ask the Hard Question of World 2: when a mind "grasps" a theory, what happens next, and what does the grasping consist of? Every answer that does real work, he would say, describes brain processes, and the separate World 2 adds nothing. On the self-refutation argument, Dennett's reply is that being caused and being moved by reasons are not rivals. A chess program is wholly physical and still plays the move it plays because it is the best move.
Eccles went further than Popper, suggesting that the mind nudges the brain by shifting the probabilities of events at synapses. Popper was more cautious. He presented interactionism as a conjecture, open to criticism, as every theory should be. That is a stance Dennett could respect, even while betting heavily against it.
| Popper | Dennett | |
|---|---|---|
| What is the mind? | An evolved, emergent World 2 that interacts with the brain and links it to World 3. | What the brain does: a pattern of functions. |
| Is phenomenal consciousness real? | Yes. Conscious states are real and make a causal difference. | No, not as philosophers define it. It is an illusion produced by the brain. |
| Could a machine have a mind? | Doubtful: describing and arguing are more than any physical theory explains. | Yes, in principle, if it does what we do. |
| Is there a hard problem? | An open problem; interactionism is offered as a conjecture. | No. There is the Hard Question: and then what happens? |
Further reading
- Karl R. Popper and John C. Eccles, The Self and Its Brain: An Argument for Interactionism (Springer, 1977).
- Karl R. Popper, Objective Knowledge: An Evolutionary Approach (Oxford, 1972), for the three worlds.
- Karl R. Popper, Conjectures and Refutations (Routledge, 1963): the best way into his philosophy of science.
- Daniel C. Dennett, Kinds of Minds (Weidenfeld & Nicolson, 1996), for the Tower of Generate-and-Test.
- Karl Popper, Stanford Encyclopedia of Philosophy.