Definite Descriptions

Sometimes we want to refer to an individual by a description rather than a name. When a description uniquely picks out an individual, it's called a definite description. Definite descriptions are of the form "the so-and-so". Examples include "the present King of France", "the tallest man in London" and "the person with the shock of blue hair."  An indefinite description, in contrast, doesn't definitely pick out an individual. "A tall story" doesn't uniquely pick out a tall story. "The tall story" does. We'll see that we can use identity to specify a unique referent for a definite description.

We'll consider definite descriptions within propositions. Here's an example:

The definite description is "the man in the overcoat."  We use existential quantification to express that there is a man in an overcoat, and we add that if anything else is a man in an overcoat, then it is identical to the man we're talking about.  Let's look at the translation:

We read this as: there is an x, such that x is a man and x wears an overcoat, and for all y, if y is a man and y wears an overcoat, then x is identical to y, and x is a spy.

This sentence, announcing the exercise on definite descriptions,  contains a definite description.