It is argued that traditional functional explanations of the incest taboo do not sufficiently supply causal conditions. It is widely acknowledged that the incest taboo, although universal among human societies, is largely a feature of human behavior. Husserl's investigations of intentionality are introduced to supply the particularly human element by which the taboo may be understood. So as to illumine the contrast between the conflicting intentionalities, a classical Aristotelian contrast between eros and parent/ child philia is drawn. Parent/child philia and eros, although both requisite for the survival of the species, are shown to be crosspurposeful in several ways. Husserl's understanding of 'negation,' as it applies to affective and practical intentionalities, is reconsidered. It is thereby explained how parental/child affection and erotic love, are 'incompossible' and not only conflict, but 'nullify' and 'cancel out' each other, generating the 'taboo'.