The context principle sets out the idea that individual words have no meaning or value unless they are understood within the context of a sentence. The notion has also been called Frege's Principle after its inventor, Gottlob Frege, contextualism, and reverse compositionality. The principle is an integral part of how words and sentences gain meaning, and which of the two is the most important in determining it.
The idea first appeared in Frege's Foundations of Arithmetic in 1884. In this book, he set out his three fundamental principles of philosophical analysis. These principles were to separate the subjective from the objective, to never find a meaning in a word without context, and to remember what divides a concept from an object. Frege created the context principle as a reaction against the atomization of meaning as posited by psychologism and compositionality.
Pyschologism is the inclusion of logic and psychology in philosophy. While this idea is mostly German in origin, John Stuart Mill was also a key proponent. Psychologism and compositionality hold that a sentence's meaning is understood from the combined meanings of its individual parts. This means that each word contributes value to the overall meaning.
Reverse compositionality or the context principle takes a diametrically opposite view: a word by itself has no real meaning. Its meaning is gained from its context within a sentence. This does not mean that every word varies in meaning from sentence to sentence, but some, like "set," do.
Frege only mentioned the context principle on a few occasions within the book and never elaborated on its meaning. It is not even certain whether Frege continued to believe in the principle or whether he diluted it or abandoned it altogether in later life. What is known is that Ludwig Wittgenstein and Bertrand Russell took on his ideas and developed them further.
Wittgenstein elaborated on Frege's principle by dividing language into propositions and propositional variables. The proposition is the sentence. The sentence is built by a series of variables or words, but it is the proposition that ultimately determines the value of each variable.
There are two versions of contextualism in this principle. In one view, a word's meaning is only determined from all of its contexts. In the other, a word's meaning can be determined from any one context. The same principles are also applied to the meanings of expressions.
The idea of the whole determining the meaning of the constituent part makes the context principle a part of semantic holism. Semantic holism is a linguistic principle that believes the meanings of sentences and words are derived from a wider context. While this wider context is not defined, it is commonly understood to mean the whole language.
Taking the context principle and semantic holism at face value creates a problem for language learners. This is because, if a language is to be learned, the learner will have to understand the whole language in order to understand one word or sentence within it. This may be impossible, as language learners build knowledge through the acquisition of individual words and phrases while slowly coming to understand the whole.