Four Stages:

1. Sensorimotor Stage (birth to 2 years) - "knows" through active interaction with environment; becomes aware of cause and effect relationships; learns that objects exist even when not in view; imitates crudely the action of others.

2. Pre-operational Stage (ages 2 to 6 years) - begins by being very egocentric; language and mental representations develop; objects are classified on just one characteristic at a time.

3. Concrete Operations Stage (ages 7 to 12 years) - develops conservation of volume, length, etc.; organizes objects into ordered categories; understands rational terms (i.e. bigger than, above); begins using simple logic.

4. Formal Operations Stage (ages over 12) - tThinking becomes abstract and symbolic; reasoning skills and a sense of hypothetical concept develop.

These stages apply to all individuals and indicate a qualitative difference.

Major Concepts:

1. Accommodation - the process of altering or revising an existing schema in light of new information.

2. Assimilation - the process of adding new material or information to an existing schema.

3. Object Permanence - the appreciation that an object no longer in view can still exist and reappear later (this occurs early during stage two).

4. Schema - system of organized general knowledge stored in longterm memory, that guides the encoding and retrieval of information.