Sense:
* The conceptual content of an expression. It's the internal, mental representation of the meaning.
* Think of it like a definition or a description.
* Example: The sense of "the current president of the United States" is something like "the person who holds the highest elected office in the United States government."
Reference:
* The actual thing in the world that an expression points to. It's the external, concrete entity.
* Example: The reference of "the current president of the United States" is currently Joe Biden.
Why Not Every Expression Has Reference:
* Abstract concepts: Expressions referring to abstract ideas, like "justice," "love," or "infinity," don't have a specific, concrete reference.
* Fictional entities: Characters in stories or myths, like "Sherlock Holmes," have a sense (a description of their personality and traits) but don't actually exist in the real world.
* Future or hypothetical things: Sentences like "The next president of the United States" have a sense but the reference is unknown until that person is elected.
* Propositions: Sentences like "All cats are mammals" express a truth, but they don't refer to any particular cat or mammal.
Key takeaway:
An expression can have a clear sense (meaning) without having a definite reference (real-world thing). This is because sense is about the conceptual content, while reference is about the actual thing being pointed to.