Piaget's theory of sensori-motor development accords a central role to action, since it is through action that infants construct knowledge of the world. However, recent evidence of high level perception in young infants suggests that Piaget was at least wrong about the nature of knowledge that infants develop. They appear to perceive the objective nature of the world from birth and it is likely that the knowledge they develop has to do with the relationship between the structure of their surroundings and the structure of their acts. This possibility is evaluated in relation to two motor developments, achievement of the sitting posture and onset of locomotion. Data are reviewed pointing to causal relationships between crawling experience and visual cliff avoidance. Additionally, data are reviewed indicating a suggestive temporal relationship between these developments and specific spatial orientation abilities. Finally, a theoretical framework is developed as an attempt to conceptualize the different types of mental development that may relate to specific motor developments.