The fundamental relations that underlie cognitive comparisons -- "same" and "different" -- can be defined at multiple levels of abstraction, which vary in relational complexity. We compared reaction times to decide whether or not two sequentially-presented perceptual displays were the same at three levels: perceptual, relational, and system (higher-order relations). For both 150 msec and 5 sec interstimulus intervals, decision time increased with level of abstraction. Sameness at lower complexity levels contributed to decisions based on the higher levels. Relations at multiple levels of complexity can be abstracted and compared in working memory, with higher complexity levels requiring more processing time. Multiple levels can cooperate to reach a decision.