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In Piaget's theory, concrete operational thought is characterized by serial ordering, simple classification, and conservation logic applied directly to objects. A person using concrete reasoning doing a Piagetian task must be able to observe objects and/or manipulate them. Formal operational thought involves proportional reasoning, separations or variables, elimination of contradictions, and class inclusion of exclusion operations. A person using formal reasoning is able to work in situations where one does not deal directly with tangible objects. A person using formal reasoning can apply concrete operations and can go beyond these operating when solving problems.