Past the arbitrary sign and synchrony and diachrony, the core of the Course: linguistic value (valeur linguistique). Part 2, chapter 4.

1. Meaning (signification) is not value (valeur)

A word has two faces. One is “paired with a concept” (meaning). The other is “set against, and compared with, other words” (value). Saussure explains it with a coin.

a word exchange (unlike) an idea compare (like) = other words a coin: exchanged for bread, compared with a 1-franc coin or a dollar. A word is the same. Meaning alone does not fix its value.
A coin's value is fixed by "what it exchanges for (bread)" and "what like thing it compares with (1 franc, 1 dollar)." A word likewise. Diagram: L/LAB

To know what a five-franc coin is worth, Saussure says, you must know both that it can be “exchanged for a different thing (échanger contre une chose différente), e.g. bread,” and that it can be “compared with a similar value (comparer avec une valeur similaire)” — a one-franc coin, a dollar. “Likewise a word can be exchanged for something dissimilar — an idea — and can be compared with something of the same kind — another word” (Part 2, ch. 4; French quotations rendered by L/LAB throughout). Its content “is really determined only by the concurrence of what exists outside it.”

2. mouton ≠ sheep

the field "sheep/mutton" mouton (Fr.) sheep (Eng.) mutton (Eng.) the live animal the meat on the table Meanings can overlap. But sheep, with mutton beside it, covers less ground than mouton — a different value.
What French covers with one word, mouton, English splits between sheep and mutton. Same meaning, different value. Diagram: L/LAB

Le français mouton peut avoir la même signification que l’anglais sheep, mais non la même valeur … parce qu’en parlant d’une pièce de viande apprêtée et servie sur la table, l’anglais dit mutton et non sheep.

French mouton can have the same meaning as English sheep, but not the same value … because, in speaking of a piece of meat dressed and served at table, English says mutton, not sheep. (Part 2, ch. 4)

The same holds for synonyms. French redouter, craindre, avoir peur (“to fear”) have content only through their mutual opposition; if one disappeared, its content would flow into the others.

3. “In a language there are only differences”

ideas (in themselves a formless continuum) sound (in itself a formless continuum) the system cuts top and bottom "at once"
Neither the continuum of ideas nor the continuum of sound has divisions of its own. The system cuts both at once, and only differences remain. Diagram: L/LAB

dans la langue il n’y a que des différences. Bien plus : une différence suppose en général des termes positifs entre lesquels elle s’établit ; mais dans la langue il n’y a que des différences sans termes positifs.

In a language there are only differences. More than that — a difference generally presupposes positive terms between which it is set up; but in a language there are only differences without positive terms. (Part 2, ch. 4)

Take the signified or the signifier: there are no ideas or sounds that exist prior to the system. There are only conceptual differences and phonic differences issuing from it. “The value of a term can be altered without touching its sense or its sounds — merely because a neighbouring term has changed.”

4. But the sign taken as a whole is positive

Here Saussure adds an important reservation. To say “everything in a language is negative” is true only of the signified and the signifier taken separately. “As soon as one considers the sign in its totality, one is in the presence of something positive in its order” (une chose positive dans son ordre, Part 2, ch. 4). Signified and signifier are each purely differential and negative, but their combination is a positive fact. And between two full signs there is no longer “difference” but “opposition”: père and mère are not “different,” they are “distinct.”

5. A sheet of paper

Saussure’s last image: “a language is comparable to a sheet of paper: thought is the front and sound is the back; you cannot cut the front without at the same time cutting the back” (on ne peut découper le recto sans découper en même temps le verso, Part 2, ch. 4). Thought in itself is a formless fog; sound in itself is a formless stream. A language is “a series of contiguous subdivisions traced at once” on both. So you cannot lift meaning out of sound.

6. The tie to Sapir’s “phoneme”

“A unit with no positive content of its own, fixed only by its oppositions in the system” — this is exactly the definition of what Sapir called the ‘ideal sound pattern’ in 1921, and what became the phoneme. The Prague School (Trubetzkoy, Jakobson) gave this “value by opposition” its most precise form in the domain of sound.

Saussure (1916, in French) and Sapir (1921, in English), independently, reached the same conclusion: a unit of language is not a substance but a position in a system. That convergence is the starting point of twentieth-century “structuralist” linguistics.

In sum

Meaning ≠ valuemeaning = the pairing with a concept; value = opposition to and comparison with other words
mouton ≠ sheepjust having mutton beside it changes the value of sheep
In a language, only differences without positive termsneither ideas nor sounds exist prior to the system
But the sign as a whole is positivebetween full signs it is not “difference” but “opposition”
A sheet of paperthought and sound are cut at the same stroke

The Saussure feature: the person and the birth of linguistics / overview / explained one, two / langage, langue, parole / the arbitrary sign / synchrony and diachrony / this essay. See also his contemporary, Sapir.