Cognitive psychology made slow progress in the early years due to the growing influence of behaviourism, an approach which constrained psychologists to the investigation of externally observable behaviour. The behaviourist position was clearly stated by Watson, who maintained that psychologists should consider only events that were observable, such as the stimulus presented and any consequent behavioural response to that stimulus. The schema theory proposes that all new perceptual input is analysed by comparing it with items which are already in our memory store, such as shapes and sounds which are familiar from past experience. These stored items are referred to as ‘schemas’, and they include a huge variety of sensory patterns and concepts. Schema and Gestalt theory had a major influence on the development of cognitive psychology, by emphasising the role played by inner mental processes and stored knowledge, rather than considering only stimulus and response.