What follows is a summary of basic principles pertaining to the definitions used in constructing an ontology. A definition is a statement of necessary and sufficient conditions. What this means in the simplest case can be understood as follows. To say that ɸ‐ing is a necessary condition for being an A is just another way of saying that every A ɸ’s; to say that ɸ‐ing is a sufficient condition for being an A is just another way of saying that everything (...) that ɸ’s is an A. The goal in writing a definition is to specify a set of conditions of this sort which are all necessary, and which are jointly sufficient. (shrink)