In short
Gottlob Frege gave Wittgenstein modern logic, the distinction between sense and reference and the principle that a word only has meaning in the context of a sentence. The later Wittgenstein rejected Frege's demand that concepts have sharp boundaries and his picture of meaning as a timeless object.
Gottlob Frege (1848-1925) was the mathematician from Jena who invented modern logic, and the thinker Wittgenstein respected more than almost anyone. In 1911 the young engineering student travelled to Jena to talk about the foundations of mathematics. Frege argued him into the ground, then pointed him to Bertrand Russell in Cambridge.
What Wittgenstein took from Frege
Three things above all. First, a new logic. Frege's Begriffsschrift (1879) analysed sentences as functions and arguments instead of subject and predicate, and that is the toolkit the Tractatus works with. Second, a seriousness about meaning. Frege's distinction between sense (Sinn) and reference (Bedeutung) explained how "the morning star" and "the evening star" can name the same planet and still say different things. Third, the context principle from Die Grundlagen der Arithmetik (1884): never ask for the meaning of a word in isolation, only in the context of a sentence. The Tractatus repeats it almost word for word.
Nur der Satz hat Sinn; nur im Zusammenhange des Satzes hat ein Name Bedeutung.
Only the proposition has sense; only in the context of a proposition does a name have meaning.
Tractatus 3.3
The preface of the Tractatus thanks "Frege's magnificent works" and the writings of Russell for much of the stimulus behind it. Frege, who read the typescript in 1919, replied that he could hardly understand it.
Where Wittgenstein turned against him
Even in the Tractatus Wittgenstein drops a lot of Frege's machinery. For Frege, sentences refer to truth values and the laws of logic are the most general truths there are. Wittgenstein answers that the propositions of logic are tautologies that say nothing (Tractatus 6.1) and that the logical constants do not stand for anything (Tractatus 4.0312).
In the later work the break goes deeper. Frege held that a concept without sharp boundaries is no real concept at all. The Philosophical Investigations reply with the example of games:
Man kann sagen, der Begriff ‚Spiel' ist ein Begriff mit verschwommenen Rändern.
One can say that the concept "game" is a concept with blurred edges.
PI §71
Blurred is not the same as useless. "Stand roughly over there" is a perfectly good instruction. That thought grows into family resemblance. Wittgenstein also takes apart Frege's assertion sign (PI §22) and the idea that a sense is an abstract object the mind grasps, and puts the use of words in its place.
Why Frege still matters for reading Wittgenstein
Many targets in the Investigations are Frege's views, sometimes unnamed. Readers who know a little Frege see that much of the quarrel with "the philosopher" is a quarrel with the logician Wittgenstein admired most. How much of Frege survives into the later work is disputed: Cora Diamond and other resolute readers argue that the context principle stays central throughout, while others see a clean break. The Stanford Encyclopedia entry on Frege and the Internet Encyclopedia of Philosophy are good places to start.
Frequently asked questions
Did Wittgenstein ever meet Frege?
Yes. He first visited Frege in Jena in 1911 and saw him again a few times before the First World War. They corresponded until about 1920, but Frege never warmed to the Tractatus.
What is the main difference between Frege and the later Wittgenstein on concepts?
Frege demanded that every concept have sharp boundaries. The later Wittgenstein argued that many useful concepts, like 'game', have blurred edges and are held together by family resemblance rather than a definition.
Sources
wittgenstein.be, “Gottlob Frege”, https://wittgenstein.be/en/philosophers/gottlob-frege/ (2026-10-20)