The extension is simply the collection of individuals that share these characteristics. Of course, later in his career Durkheim would come to identify the meanings of general concepts with collective rather than individual representations. However, this was merely to introduce a distinction between two sorts of representative ideas. The metaphysical views Durkheim endorsed in lectures 70 through 72 may also shed light on his subsequent sociological theorizing. For the spiritualist philosophy, which he traced back to Leibniz and also associated with vitalism, reality consists in forces or spirits analogous to ourselves, but with perhaps with less consciousness.
Nothing exists except either as a spirit or as an object of representation.
Thus, unlike Cartesian dualism, spiritualism is a monist realism that is not faced with the problem of how two essentially different sorts of substance, mind and matter, are able to interact with each other. There appear, then, to be two major differences between Durkheim's early spiritualism and his later social realism. First, he seems to have added social forces to his basic ontology. Second, he came to accept that there are two distinct classes of mental representations, individual and collective, that constitute the subject matters of psychology and sociology, respectively. However, this notion of a collective representation is already suggested by the conceptualist notion of shared mental states.