The sensation-perception distinction did not appear before the seventeenth century, but since then various formulations of it have gained wide acceptance. This is not an historical accident and the article suggests an explanation for its appearance. Section 1 describes a basic assumption underlying the sensation-perception distinction, to wit, the postulation of a pure sensory stage--viz. sensation--devoid of active influence of the agent's cognitive, emotional, and evaluative frameworks. These frameworks are passive in that stage. I call this postulation the passivity assumption. (...) Section 2 suggests three major reasons for the emergence of this assumption in the seventeenth century: the mental-physical gap, the causal theory of perception, and epistemological considerations regarding the status of the sensory given. In the last section a critical discussion is presented. The passivity assumption is found to have serious empirical and theoretical flaws. (shrink)