This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Gottlob Frege's 1892 essay "On Sense and Reference" opened a problem that occupies philosophy of language to this day, namely how to explain why two ways of referring to the same thing can carry such different cognitive weight. The puzzle begins with identity statements. The morning star is the morning star is knowable just by inspecting the sentence, true by logical form alone. The morning star is the evening star, by contrast, states something that took centuries of astronomical observation to establish, namely that Venus appears both at dawn and at dusk. Yet both sentences, on the usual view of meaning current before Frege, say the same thing, since the morning star and the evening star pick out the same planet. If meaning were exhausted by what a term denotes, the two identity claims would be equally trivial. They plainly are not, and Frege took that asymmetry as proof that reference cannot be the whole story.
His solution was to split the meaning of an expression into two components. Its reference, or Bedeutung, is the object it denotes, Venus in this case. Its sense, or Sinn, is the particular way that object is presented, the route of thought by which a speaker arrives at it. Morning star presents Venus as the last bright point visible before sunrise, evening star presents the same planet as the first visible after sunset. Same Bedeutung, different Sinn, and it is the difference in sense, not any difference in reference, that makes the identity statement informative rather than empty. Frege generalized the point beyond astronomy. Four and eight divided by two denote the same number through different arithmetical routes, and ordinary proper names work the same way, so that a sentence identifying two names for one historical figure can carry real news even though, referentially, it says an object is identical to itself.
Frege pressed the distinction further than most successors have needed to. He held that sense attaches not just to names but to whole sentences. A sentence's reference, on his account, is startlingly austere, nothing more than a truth value, the True or the False, since substituting any expression for another with the same reference inside a sentence leaves the sentence's truth value unchanged. What a sentence expresses beyond its truth value, the actual content asserted, is what Frege called a thought, a Gedanke, and he identified the thought as the sense of the sentence. Thoughts are not psychological events. Frege insisted, against the psychologism common among his contemporaries, that a thought is not a mental image or an act of judging but something a mind can grasp without owning it, the way many people can grasp one and the same theorem. He placed thoughts in what he called a third realm, distinct both from the physical world of Venus and sunlight and from the private inner world of sensations and ideas, precisely because a thought had to be available to be shared, checked, and disputed by different thinkers at different times without becoming a different thought each time it was grasped. Two mathematicians proving the same theorem grasp one identical thought, not two resembling private copies of it, and this mind independence is what let Frege treat logic as a science of objective truths rather than a description of how human beings happen to reason.
The distinction also solved a puzzle about belief. John believes Mark Twain wrote Huckleberry Finn can be true while John believes Samuel Clemens wrote Huckleberry Finn is false, even though Twain and Clemens are one person, because inside a belief context a name no longer picks out its ordinary object but instead denotes its own customary sense. Since the two names carry different senses, they behave, inside belief reports, like names of different things, and substitution fails exactly where a theory of meaning built on reference alone could not explain why it should. The sense and reference distinction thereby did far more than resolve a puzzle about Venus, it supplied the machinery later philosophy of language still uses to explain how thought and language can be both about a shared, mind independent world and, at the same time, informative, mistaken, and disputable.