Armstrong holds to a physicalist, functionalist theory of the mind. He initially was attracted to Gilbert Ryle's "The Concept of Mind" and the rejection of Cartesian dualism. Armstrong did not accept behaviourism and instead defended a theory he referred to as the "central-state theory" which identifies mental states with the state of the central nervous system. In "A Materialist Theory of Mind", he accepted that mental states such as consciousness exist, but stated that they can be explained as physical phenomena. Armstrong attributes his adoption of the central-state theory to the work of J. J. C. Smart—specifically the paper 'Sensations and Brain Processes'—and traces the lineage from there to Ullin Place's 1956 paper 'Is Consciousness a Brain Process?'