Frege's Distinction Between Sense and Reference
Frege's distinction between sense and reference is a foundational concept in semantics. It explains how two expressions can mean different things yet pick out the same object in the world. This matters because it resolves real puzzles about language: why "Clark Kent is Superman" feels informative while "Superman is Superman" feels trivial, even though both are about the same person. Understanding this distinction is key to seeing how language connects to both our thoughts and the external world.
Sense vs. Reference
Frege introduced two German terms that map onto two layers of meaning every expression carries:
- Sense (Sinn) is the cognitive content or mode of presentation of an expression. It captures how the referent is described or presented. For example, "the morning star" presents its referent as the bright celestial body visible before sunrise.
- Sense is what determines the thought or proposition a sentence expresses. It's the informative content you grasp when you understand an expression.
- Reference (Bedeutung) is the actual object, entity, or value that an expression picks out in the world. For "the morning star," the reference is the planet Venus itself.
- Depending on the type of expression, a reference can be an individual object, a truth value (for whole sentences), or something else entirely.
The crucial relationship between them: sense determines reference by specifying the conditions for identifying the referent, but reference does not determine sense. That's why multiple senses can pick out the same referent. Both "the morning star" and "the evening star" have different senses (different modes of presentation), yet they refer to the same object: Venus.
Examples of the Distinction
These examples help make the abstract idea concrete:
- "The author of Waverley" vs. "Sir Walter Scott": Different senses, same reference. The first presents the referent as the person who wrote a specific novel; the second presents the referent by proper name. Both pick out the same individual.
- "2 + 2" vs. "4": Different senses, same reference. "2 + 2" presents the referent as the result of an arithmetic operation; "4" presents it directly as a number. Both denote the same value.
- "The largest planet in the solar system" vs. "Jupiter": Different senses, same reference. The first identifies the referent by a descriptive property (relative size within a location); the second uses a proper name.
In each case, you could learn something new by discovering that the two expressions co-refer, precisely because their senses differ.
Impact on Expression Meaning
The sense/reference distinction has direct consequences for how substitution works in sentences:
- In transparent (extensional) contexts, swapping one co-referential term for another preserves truth value. If "the morning star is visible in the sky" is true, then "the evening star is visible in the sky" is also true, since both refer to Venus.
- In opaque (intensional) contexts, like belief reports, substitution can change truth value. "John believes the morning star is visible" can be true while "John believes the evening star is visible" is false. John might not know the two names refer to the same planet, so his beliefs track sense, not just reference.
Frege also applied the distinction to complex expressions:
- The sense of a whole sentence is built from the senses of its parts and how they combine. The sense of "The morning star is bright" depends on the senses of "the morning star," "is," and "bright."
- The reference of a whole sentence (which Frege took to be its truth value) depends on the references of its parts. Whether "The morning star is bright" is true or false depends on whether Venus (the reference of "the morning star") actually has the property of being bright.
Philosophical Implications
Frege's distinction resolves several important puzzles:
- The puzzle of identity statements. "Hesperus is Hesperus" is trivially true and uninformative. "Hesperus is Phosphorus" is also true (both names refer to Venus), but it's genuinely informative. The difference in cognitive significance comes from the different senses of "Hesperus" and "Phosphorus," even though their reference is identical.
- Necessary vs. contingent truths. Some truths hold by virtue of the senses involved: "2 + 2 = 4" is necessarily true because of what those expressions mean. Other truths depend on how the world happens to be: "Aristotle taught Alexander the Great" is contingently true, since things could have gone differently.
- A two-level theory of meaning. Sense captures the cognitive role of an expression (how it figures in reasoning and understanding). Reference captures its truth-conditional contribution (how it connects to the world). Together, they give a richer account of meaning than either layer could provide alone.