In Piaget's first cognitive stage (birth to ~2 years), infants learn about the world through sensory input and motor action. The infant progresses through six substages, from reflexive responses to intentional behavior to early symbolic thought. The landmark achievement is object permanence — understanding that objects continue to exist when out of sight — which develops gradually through the first year. By the end of the sensorimotor stage, children begin to use mental representations, enabling deferred imitation and early pretend play.
Observe or review A-not-B error experiments (infants search where they previously found an object) to see object permanence developing in real time. Timeline the six substages with their key milestones.
From your prerequisite in developmental psychology, you know that Piaget viewed cognitive development as the construction of increasingly sophisticated schemas — organized patterns of thought or action that the mind uses to interpret experience. The sensorimotor stage is where this construction begins, from scratch, in the newborn. The infant has no mental representations yet; knowledge exists only in the doing. To know a rattle is to shake it, mouth it, drop it and watch it fall. The world is constituted entirely through sensory input and motor action — hence the stage's name.
Piaget divided the sensorimotor stage into six substages that trace a remarkable progression. In the first substage (0–1 month), behavior is purely reflexive: sucking, grasping, rooting — innate responses that fire without learning. From the second substage onward, the infant begins to repeat actions that produce interesting results, a pattern Piaget called a circular reaction. A primary circular reaction (1–4 months) centers on the infant's own body — thumb-sucking repeated for its own pleasure. A secondary circular reaction (4–8 months) turns outward: the infant bats a mobile and then bats it again because something interesting happened in the world. By the fifth substage (12–18 months), the infant engages in tertiary circular reactions — deliberately varying actions to discover what happens, like a scientist running small experiments. The sixth substage (18–24 months) marks the transition to mental representation: the infant can imagine actions before performing them, enabling deferred imitation (copying behavior seen hours earlier) and early pretend play.
The landmark achievement of the stage is object permanence — the understanding that objects continue to exist when they are no longer perceived. This seems obvious to adults because we have internalized it completely, but it is genuinely constructed over the first two years. A four-month-old whose toy is hidden under a cloth will not search for it; it is as if the toy has ceased to exist. By eight to twelve months, infants search for hidden objects — but commit the A-not-B error: if they repeatedly find an object at location A and then watch it hidden at location B, they still search at A. This error reveals that object permanence is still tied to the infant's own action history rather than an abstract representation of the object's location. Full object permanence — searching correctly even after invisible displacements — is not achieved until late in the first year.
One important nuance: Piaget's timetable was based on observational research with his own children, and later researchers using more sensitive methods found competence emerging earlier than he proposed. Renée Baillargeon's violation-of-expectation studies showed that infants as young as 3–4 months look longer at physically impossible events (an object that should be blocked reappears from behind a screen), suggesting implicit knowledge of object permanence well before they can act on it. This means Piaget conflated the age at which knowledge is explicit enough to guide search behavior with the age at which some understanding exists. The underlying idea — that knowledge is constructed through action and experience — remains the stage's enduring contribution.