 The human brain and nervous system is not much like a digital computer. In particular, there are reasons for being skeptical of the claim that the brain is a discrete-state machine. Turing observes that a small error in the information about the size of a nervous impulse impinging on a neuron may make a large difference to the size of the outgoing impulse. From this, Turing infers that the brain is likely to be a continuous-state machine; and he then notes that, since discrete-state machines are not continuous-state machines, there might be reason here for thinking that no discrete-state machine can be intelligent.