This study begins by describing how actor-network theory conceives: the failure of the project of modernity and its old dualisms, the death of the social as a solid and fundamental explanatory factor, and the birth or rediscovery of the post-social or post-humanist world. It goes on to describe how the theory proposes, as the main alternative, that the principle of symmetry of socio-constructivist sociology be generalised, using relational semiotics that includes all actants involved in each situation, whether human or non-human. It then discusses the most notable excesses, insufficiencies and ambiguities of this theory, present in both its diagnoses and proposals. Hence the study seeks to confirm that sociology and other social sciences have reasons to understand and indeed learn from this original and innovative theory, but that they also have many reasons to disagree, contradict and indeed to reaffirm their own positions.