The Self and Its Brain: An Argument for Interactionism

The central argument is that the self is a mental entity distinct from the brain, and that it can causally influence physical states—a position known as interactionist dualism. Popper and Eccles reject both reductive materialism (the view that mental states are nothing but brain states) and epiphenomenalism (the view that mental states are byproducts of brain activity with no causal power). Their alternative is a three-world ontology: World 1 is the physical world, World 2 is the world of subjective experiences, and World 3 is the world of objective contents. The self, situated in World 2, interacts with World 1 through the brain, which serves as its instrument. The book is divided into three parts: Popper's philosophical argument, Eccles's neuroscientific evidence, and a dialogue between the two authors.

Eccles argues that the supplementary motor area (SMA) and the readiness potential (Bereitschaftspotential) provide evidence for a non-physical self that initiates voluntary action. He contends that the readiness potential, which precedes conscious awareness of the intention to act, reflects the brain's preparation, not the self's decision, and that the self then acts on the brain to trigger the action. This interpretation has been widely criticised. Benjamin Libet's experiments, on which Eccles relied, are generally interpreted as showing that unconscious brain processes precede conscious decisions, which is often taken as evidence against interactionism. Eccles's dualism is a minority position in contemporary neuroscience, though it remains a significant contribution to the mind-body debate.

Popper's three-world ontology is the framework within which the argument for interactionism is set. World 1 is the physical world; World 2 is the world of subjective experiences, including the self; World 3 is the world of objective contents—theories, problems, works of art—which are embodied in World 1 but not reducible to it. World 3 plays a crucial role because it provides the contents that the self engages with in thought and action. The self, situated in World 2, interacts with World 3 (through language and culture) and with World 1 (through the brain). Popper argues that this three-world framework makes interactionism intelligible: the self is not a ghost in the machine but a genuine entity that can act on the brain because the brain is its instrument for engaging with the world.

The book was widely reviewed but not well received by mainstream philosophers and neuroscientists. The main criticism was that the interactionist dualism it defends is incompatible with the scientific worldview, and that the neuroscientific evidence Eccles cites does not support his interpretation. Critics argued that the readiness potential and other findings are better explained by materialist accounts. Some reviewers also found the dialogue between Popper and Eccles to be less illuminating than the individual contributions. Despite these criticisms, the book is a significant historical document: it shows two major thinkers engaging seriously with the mind-body problem from a non-materialist perspective, and it remains a reference point in debates about consciousness and free will.

The book extends Popper's philosophy of science into the philosophy of mind, and it is consistent with his earlier work in several ways. His three-world ontology was already developed in Objective Knowledge (1972), and his defence of indeterminism and free will is anticipated in The Open Universe. The book is not a departure but an application of his evolutionary epistemology to the mind-body problem. Popper argues that the self is a product of evolution, and that its capacity to interact with the brain is a further development of the organism's capacity to act on the world. The dialogue with Eccles allows Popper to engage with neuroscience, which he regarded as essential to a complete account of knowledge and mind.

The most common misunderstanding is that Popper and Eccles are defending a Cartesian dualism of two substances, mind and matter. In fact, they are defending a form of interactionism that is naturalistic and evolutionary: the self is not a separate substance but a product of evolution, and its interaction with the brain is a natural process. A related misunderstanding is that the book is primarily a work of neuroscience. It is a philosophical book, with Eccles supplying the neuroscientific evidence. The central argument is philosophical, and the neuroscience is used to support it. Reading the book as a contribution to neuroscience, rather than as a contribution to the philosophy of mind, misses its aim and its significance.