 There are, first of all, the problems faced by the Epicureans, who were genetic but not explanatory empiricists, favouring the atomist theory of Leucippus and Democritus. As Democritus seems to have realized, while the atomist doctrine can be supported by observable facts (such as the fact of motion), once formulated it seems to undercut a belief in the reliability of such observations. At the very least, it suggests a distinction between what would later be called “primary” and “secondary” qualities, narrowing the range of the kind of experiences to which natural philosophers would appeal.