1. in logic, the principle that where X is known to be identical to Y, any statement about X (or Y) will have the same meaning and truth value as the same statement about Y (or X). So, for example, any statement made about Paris will have the same meaning, and be equally true or false, as the same statement made about the capital of France. See also laws of thought.

2. in the theory of Jean Piaget, the principle underlying a child’s awareness of the conservation of physical quantities.