Piaget's Eras and Stages of Physical-Cognative Development

Era I (age 0-2): The era of sensorimotor intelligence

Stage 1. Reflex action.
Stage 2. Coordination of reflexes and sensorimotor repetition (primary Circular reaction).
Stage 3. Activities to make interesting events in the environment reappear (secondary circular reaction).
Stage 4. Means/ends behavior and search for absent objects.
Stage 5. Experimental search for new means (tertiary circular reaction).
Stage 6. Use of imagery in insightful invention of new means and in recall of absent objects and events.

Era II (age 2-5): Symbolic, intuitive, or prelogical thought

Inference carried on through images and symbols which do not maintain logical relations or invariances with one mother. "magical thinking" in the dens of (a) confusion of apparent or imagined events with real events and objects and (b) confusion of perceptual appearances of qualitative and quantitative charge with actual change.

Era III (age 6-10): Concrete operational thought

Inferences carried on through system of classes, relations, and quantities maintaining logically invariant properties and which refer to concrete objects. These include such logical processes as (a) inclusion of lower-order classes in higher order classes; (b) transitive seriation (recognition that if a > b and b > c, then a > c); (c) logical addition and multiplication of classes and quantities; (d) conservation of number, class membership, length, and mass under apparent change.

Substage 1. Formation of stable categorical classes.
Substage 2. Formation of quantitative and numerical relations of invariance.

Era IV (age 11 to adulthood): Formal operational thought

Inferences through logical operations upon propositions or "operations upon operations." Reasoning about reasoning. Construction of systems of all possible relations or implications. Hypothetico-deductive isolation of variables and testing of hypotheses.

Substage 1. Formation of the inverse of the reciprocal. Capacity to form negative classes (for example, the class of all not-crows) and to see relations as simultaneously reciprocal (for example, to understand that liquid in a U-shaped tube holds an equal level because of counterbalanced pressures).
Substage 2. Capacity to order triads of propositions or relations (for example, to understand that if Bob is taller than Joe and Joe is shorter than Dick, then Joe is the shortest of the three).
Substage 3. True formal thought. Construction of all possible combinations of relations, systematic isolation of variables, and deductive hypothesis-testing.

Adapted from Kohlberg and Gilligam. Evolutionary Truces. 1972.