 Intentionality (with a ‘t’) should not be confused with intensionality (with an ‘s’). The latter is another technical expression of logic and philosophy; intensions are opposed to extensions. Russell’s example is that the intension of ‘featherless biped that is not a plucked chicken’ is quite different than the intension of ‘human being,’ although their extensions are (perhaps) the same. Extensions include classes, sets, and functions considered as sets of ordered pairs and truth-values. Intensions are meaning, rules, properties, functions as ordinarily conceived, and propositions. In possible-worlds semantics intensions are often identified with sets of possible worlds or functions that take possible worlds as arguments. A property, for example, might be identified with a function from worlds to set of objects (those that have the property in the world), and a proposition with a set of worlds (those in which it is true). Whether such an identification is meant as a reduction or explanation of intensions as extensions, or simply as a way of modeling intensions as extensions, varies from theorist to theorist.