Since Descartes it has been customary to divide reality into res extensa (the extended, the physical, the body) and res cogitans (the mental, the mind). This assumption is now recognized to have extremely problematic effects, yet no satisfactory substitute is available. Among the problems generated by the Cartesian dichotomy between the mental and the physical is the association of nature with the quantitative and measurable and the association of qualities with the “subjective.” Likewise, the dichotomy entails an unbridgeable gap between the “inner” (mental, experiential) and the “outer” (physical). Even other subjects (other I’s) are inaccessible to a single subject’s knowledge according to the Cartesian view. Finally, the Cartesian assumption restricts the theory-generating resources of biological research—for instance, comparative psychological research—too narrowly: namely, to behavior construed as entirely extensional. In the interest of preserving and explaining the possibility of forms of knowledge and objects of knowledge characteristic of biology, the study of nature in general, and human and animal activity in everyday experience, we must find a substitute set of ontological assumptions to those in the Cartesian dichotomy. This substitute set must nonetheless still explain the appearance of a dual aspect in ourselves and other living things.