A theory of pictorial perception

things and general rules, as well as to perceive concrete objects. Words and symbols (including graphic symbols and geometrical drawings) are essential for learning about properties, variables, groups, classes, and universals. Men can make propositions with symbols, and discover new ones by manipulating old ones. They can mutually exchange propositions, and formulate general laws. If this is what needs to be learned, the surrogates for them should be arbitrary and conventional. Both kinds of surrogates have their value.