 Constants and variables are the only terms in our formal language, so all of our terms are simple, corresponding to proper names and some uses of pronouns. We call a term closed if it is not a variable. In general, we use \(v\) to represent variables, and \(t\) to represent a closed term, an individual constant. Some authors also introduce function letters, which allow complex terms corresponding to: “\(7+4\)” and “the wife of Bill Clinton”, or complex terms containing variables, like “the father of \(x\)” and “\(x/y\)”. Logic books aimed at mathematicians are likely to contain function letters, probably due to the centrality of functions in mathematical discourse. Books aimed at a more general audience (or at philosophy students), may leave out function letters, since it simplifies the syntax and theory. We follow the latter route here. This is an instance of a general tradeoff between presenting a system with greater expressive resources, at the cost of making its formal treatment more complex. 