D. M. Armstrong defends the central-state identity theory according to which the brain states are the same as the mental states. Each state of the mind is causally responsible for a piece of behaviour. Hence, there is a causal relationship between the states of mind and the expressions of behaviour. The behaviour is not simply an automatic process caused by the dispositional character of the mind. Rather the brain process is identical with the mental state.

Armstrong strives for a synthesis of two conflicting theories of mind, namely, the Cartesian theory of soul-substance which considers the autonomy of the mental as real and the behaviourists’ theory which considers behaviour as the only reality. As he describes the synthesis, “My proposed synthesis is that the mind is properly conceived as an inner principle, but a principle that is identified in terms of the outward behaviour it is apt for bringing about. This way of looking at the mind and mental states does not itself entail a Materialist or Physicalist view of man, for nothing is said in this analysis about the intrinsic nature of these mental states. But if we have, as I have asserted that we do have, general scientific grounds for thinking that man is nothing but a physical mechanism, we can go on to argue that the mental states are in fact nothing but physical states of the central nervous system.”⁶ The synthesis emphasizes the scientific study of the mental phenomena. The reality of the mental phenomena is established by identifying them with the central states of the nervous system. The central nervous system brings out the physical processes involved in the organism for causing several behaviour. And behaviour is causally explainable through the identification of the underlying physical processes of the brain. So neither behaviour nor the reality of the mental is ruled out. What the identity theory of the mind tries to look at is the inner principle which is the  essential link between mind and behaviour.