The Progress of Concrete Operations
Around the years of seven or eight, the processes of concrete operational thinking begin to emerge. These operations are mental actions, derived in the first place from physical actions, which have become internal to the mind. By virtue of concrete operations, 'immediately given data' can be restructured in the mind into new forms. Contact with the environment is maintained during such mental actions, because by reversing them a return to the perceived form is always possible. Concrete operations are reversible in two ways, by the inversion of combinations (classes), and by the reciprocity of differences (relations.) Reversibility permits conservation.