According to Swiss psychologist Jean Piaget following four stages occur in cognitive development.
Infant differentiates himself from objects; gradually becomes aware of the relationship between his actions and their effects on the environment so that he contact intentionally and make interesting events last longer (if he shakes a rattle it will make a noise); learns that objects continue to exist even though no longer visible (object permanence).
Child uses language and can represent objects by images and words; is still egocentric, the world revolves around him and he has difficulty taking the viewpoint of others; taking the viewpoint of others; classifies objects by single salient features: If A is like B in one respect, must be line B in other respects; toward the end of this stage begins to use numbers and develop conservation concepts.
He becomes capable of logical thought; achieves conservation concepts on this order: number (age 6), mass (age 7), weight (age 9); can classify objects, order them in series along a dimension (such as size), and understand relational terms (A is longer than B).
He can think in abstract terms, follow logical propositions, and reason by hypothesis; isolates the elements of a problem and systematically explores all possible solutions; becomes concerned with the hypothetical, the future, and ideological problems.