Philosophy
The Logic of Indexicals and Context-Dependent Truth
Quick fact
In the 1970s, philosopher David Kaplan proposed a formal logic for indexicals that distinguishes between character (the linguistic rule) and content (the proposition expressed). This framework revealed that the sentence 'I am here now' is logically true—true in all contexts—even though the proposition it expresses is often contingent. This solved the puzzle of how indexicals can have a stable meaning yet express varying truth conditions.
Why this is interesting
The sentence 'I am here now' is always true when spoken, yet it seems to say something different depending on who says it, where, and when. How can one sentence have so many meanings while still being a single expression?
Read the full explanation
Understanding The Logic of Indexicals and Context-Dependent Truth
Think of a sentence like a template with blank slots. The indexicals 'I', 'here', and 'now' are the fill-in pieces. When you speak, the context—who is talking, what place they are in, what time it is—fills in the slots with specific referents. So 'I am hungry' said by Alice refers to Alice, and said by Bob refers to Bob. The literal meaning of the word 'I' is just a rule: 'the speaker of this context.' Similarly, 'now' means 'the time of this context,' and 'here' means 'the location of this context.' This rule is called the character. The resulting specific proposition—'Alice is hungry' or 'Bob is hungry'—is the content. The same sentence type can have different contents in different contexts, and even different truth values: 'I am in Paris' is true when said in Paris and false when said in London.
A deeper explanation
Kaplan formalized this with two levels of meaning. The character is a function from contexts to contents—a rule that takes the context parameters (agent, time, location, possible world) and returns the proposition expressed. The content is a function from possible worlds to truth values—the proposition itself. Thus indexicals are directly referential: their content is the actual referent, not a description. This explains why 'I' cannot be replaced with a description like 'the speaker of this context' without changing meaning. The character of 'I' is a constant function: in any context c, it returns the agent of c. The content of 'I am here now' in a context c is the proposition that the agent of c is at the location of c at the time of c. Since the character guarantees that this proposition is true in the context itself, the sentence is analytically true—true in every context—but not necessarily true in the sense of being true in all possible worlds. The content expresses a contingent proposition. This distinction is crucial for handling context-dependence in logic and for understanding how indexical expressions are not ambiguous but flexible.