(1) First, take the term “human.” There are many ways to define what it means to be human, but perhaps you agree with Aristotle and define a human as “a rational animal.” This is the intension of the term. The extension of the term is the totality of the rational animals (humans) which exist in reality. We see that we may have made our definition too broad, and that there exist other rational animals besides humans. Let’s, however, accept the idea that “a rational animal” is a good definition for “human.” Let’s make the term more specific and see what happens to the extension and intension of the term. Perhaps we choose to specify by using the term “female human.” Now the intension of the term increases in terms of specificity, since the new meaning of the term is “a female rational
animal.” Conversely, the extension decreases, since the number of female humans in reality is fewer than the number of total humans.
(2) Next, let’s choose a more abstract term. Let’s say we have the term “justice.” Although justice is notoriously difficult to define, let’s say that we accept the definition, : “justice is the interest of the stronger” (this is the definition was put forth by Thrasymachus, an interlocutor in Plato’s foundational work of political philosophy, Republic). Even though this is an abstract term, we can see that the same inverse principle between extension and intension applies. For instance, we can modify Thrasymachus’s definition and say that “justice” is “the interest of both the stronger and the weaker at the same time.” We can intuitively see that this new definition would occur fewer times in the world (a smaller extension), because there are few instances in which both the stronger and the weaker benefit.