Knowledge sharing is difficult. One reason is that it is hard to decide how to describe a domain in a way which suits everyone interested in the knowledge. Tackling this problem has been a central theme of the surge in ontological research over recent years. Unfortunately, getting an agreed ontology is not the end of our problems, since the way we represent knowledge is intimately linked to the inferences we expect to perform with it. We introduce three inference systems and discuss the problems of having knowledge passing through them, which are representative of complex problems we may need to solve for knowledge sharing.