Up until the mid-1960's, Sperry strongly agreed with philosopher Karl Popper and scientist John Eccles. Like them, he rejected materialism (or physicalism) and reductionism about the mind and brain. Although Popper and Eccles described Sperry's research as supporting their idea of an as yet unexplained "interaction" between dualist mind and brain, Sperry thought of himself as a monist. Sperry in 1966 began referring to himself as a "mentalist." He suggested that the opposing view of "materialism" needed a refined definition, arguing that "mentalism" was not synonymous with "dualism" and "physicalism" was not the equivalent of "monism."

He said:

By our current mind-brain theory, monism has to include subjective mental properties as causal realities. This is not the case with physicalism or materialism which are the understood antitheses of mentalism, and have traditionally excluded mental phenomena as causal constructs. In calling myself a ‘mentalist’, I hold subjective mental phenomena to be primary, causally potent realities as they are experienced subjectively, different from, more than, and not reducible to their physicochemical elements. At the same time, I define this position and the mind-brain theory on which it is based as monistic and see it as a major deterrent to dualism.

("Mind-Brain Interaction: Mentalism, YES; Dualism, NO" in Neuroscience Vol. 5. p.196)

This would be a "new psychophysical interactionism," not the dualist interactionism of Karl Popper and John Eccles. This raises René Descartes' classical mind-body problem. Today it is described as the problem of mental causation. How can an immaterial "mental event" possibly be the cause of a material "physical event?"

In 1965 Sperry gave a lecture at the University of Chicago on "Mind, Brain, and Humanist Values." He says that he

worked the new mind-brain ideas into a discussion of holist-reductionist issues, emergent downward control and ‘nothing but’ fallacies in human value systems, in a broad refutation of the then prevalent ’mechanistic, materialistic, behavioristic, fatalistic, reductionistic view' of the 'nature of mind and psyche'. It was on this occasion that I openly changed my alignment from behaviorist materialism to antimechanistic and nonreductive mentalism (- as the term ‘mentalism’ is used in psychology in contrast to behaviorism; not, of course. in the extreme philosophic sense that would deny material reality).

(Ibid.. p.196)

Sperry gave the subjective experience of consciousness (regarded as an emergent property of brain activity) a causal role in the control of brain function. This differed from earlier emergence theories of consciousness (e.g., C. Lloyd Morgan), which were parallelistic, double aspect, or epiphenomenal. All rejected any direct causal influence of mental qualities on neural processing. Sperry's causal conscious mind strongly opposed the then dominant behaviorist philosophy of reductive mechanistic materialism. (Ibid.. p.197)

The resulting "cognitive revolution" of the early 1970's marked the return of many insights about the mind offered by William James in his 1890 Principles of Philosophy. Psychologists John Watson, B.F. Skinner, and the British philosopher Gilbert Ryle had imposed a half-century taboo on "mind" in American psychology. Although the taboo began to lift, the new "cognitive scientists" continued with a mostly mechanical and reductionist view of their mind models, as did the philosopher Daniel Dennett.

Although Karl Popper did not think indeterminism was important for free will until late in 1977, Popper did think it played a role in the mind being an originator of new causal chains. And Popper said that his two-stage model of free will was an example of downward causation! He cited both Sperry and Donald Campbell as the source of the idea of downward causation.

Sperry, on the other hand, accepted downward causal control, but denied indeterminism. He said:

Another main theme of Popper’s philosophy, indeterminism, is applied to the mind-brain relation. In this we are in fundamental disagreement. I hold that every time the elements of creation, whether atoms or concepts, are put together in the same way under the same conditions, that the same new properties would emerge and that the emergent process is, therefore causal and deterministic. To this extent and in this sense it may also be said to be, in principle, predictable though generally, with few exceptions, it is not so in practice. Rather than viewing the mind of man as a ‘first cause’ or ‘prime mover’ (Popper,1962; Popper & Eccles,1977). I see the brain as a tremendous generator of emergent novel phenomena that then exert supercedant control over lower-level activities. The higher-level functional entities of inner experience have their own dynamics in cerebral activity and, contrary to Popper’s interpretation of my view (Popper & Eccles,1977, p. 209), they also ‘interact causally with one another at their own level as entities’ (Sperry, 1969b). But the creative process is not indeterminant. The laws of causation are nowhere broken or open (excepting perhaps in quantum-level indeterminacy which is here irrelevant). It is all part of a continuous hierarchic manifold, a one-world continuum.

On these terms, human decision-making is not indeterminant but self-determinant. Everyone normally wants to have control over what he does and to determine his own choices in accordance with his own wishes. This is exactly the kind of control our mind-brain model provides (Sperry,1976b; 1977b). But this is not freedom from causal determinacy. A person may be relatively free in this view from much that goes on around him, but he is not free from his own inner self. The emphasis here is the diametric converse of the behaviorist contention that ‘ideas, motives, and feelings have no part in determining conduct and therefore no part in explaining it’

(Ibid.. p.200)

Once generated from neural events, the higher order mental patterns and programs have their own subjective qualities and progress, operate and interact by their own causal laws and principles which are different from and cannot be reduced to those of neurophysiology... The mental entities transcend the physiological just as the physiological transcends the [cellular], the molecular, the atomic and subatomic. etc. The mental forces do not violate, disturb or intervene in neuronal activity, but they do supervene. Interaction is mutually reciprocal between the neural and mental levels in the nested brain hierarchies.

(Ibid.. p.201)

Sperry says his "concept of the mind-brain relation not only refutes the doctrines of behaviorism and materialism, mechanistic determinism and reductionism, as Popper and Eccles correctly infer, but also and with equal force, strongly discounts dualism."

A Modified Concept of Consciousness (1969)

Mind-Brain Interaction: Mentalism, YES; Dualism, NO (1980)