The overview sketched the “energeia” thesis; here it is close up, from the German. The question: is a language a “thing” you can point at, or an “event” that keeps happening?

1. “Product (ergon)” and “activity (energeia)”

seen as a thing the sum of a dictionary's entries the list of a grammar's rules = countable, can sit on a shelf Aristotle's term: ergon seen as an activity the carrying-out of speaking and hearing transient at every moment = uncountable, only ever in progress Aristotle's term: energeia
Humboldt borrows Aristotle's pair (product / activity) and puts language on the side of activity. Diagram: L/LAB

Die Sprache, in ihrem wirklichen Wesen aufgefasst, ist etwas beständig und in jedem Augenblicke Vorübergehendes. … Sie selbst ist kein Werk (Ergon), sondern eine Thätigkeit (Energeia).

Language, grasped in its true essence, is something continual and, at every moment, transient. … It is itself no product (ergon) but an activity (energeia). (German quotations throughout; Japanese and English are L/LAB working translations.)

“Ergon” and “energeia” are Aristotle’s pair for the finished result and the being-at-work. Humboldt applies it to language. The thing we take ourselves to be pointing at when we say “English” or “Japanese” — a stock of words and rules — is not, in fact, the language. The language exists only in the carrying-out, in what is being spoken and heard right now.

2. “The ever-repeated labour of the mind”

thought articulated sound the work of "making it able to express" anew, each time one speaks = ever-repeated (sich ewig wiederholende)
Language is the mind's unceasing work of shaping articulated sound into the expression of thought — the work, not the finished piece. Diagram: L/LAB

So what does this “activity” actually do? Humboldt’s answer is precise.

Sie ist nemlich die sich ewig wiederholende Arbeit des Geistes, den articulirten Laut zum Ausdruck des Gedanken fähig zu machen.

It is, namely, the ever-repeated labour of the mind to make the articulated sound capable of expressing thought.

Language is not the “sound.” It is not the “thought.” It is the work of making sound into a vessel for thought. And that work is not done once and finished. It is carried out afresh every time someone speaks or hears. So language is “continual and, at every moment, transient.”

3. Hence the definition of language must be “genetic”

You cannot define something that is an activity as if it were a thing.

Ihre wahre Definition kann daher nur eine genetische seyn.

Its true definition can therefore only be a genetic one.

A genetic definition (genetische Definition) grasps its object not by “what it is made of” but by “how it is produced.” Not defining a triangle as “three line segments” by its parts, but as “swing a line about a point until it closes” — by a procedure. A language, likewise, should be grasped not as a list of words and rules but as the procedure by which a speaker converts thought into sound. Dissect (zergliedern) the object into parts and the living activity dies.

4. Dictionaries and grammars are a “dead deposit”

the stream of speaking and hearing (energeia) sediment = dictionary, grammar, paradigm tables ("dead deposit")
Dictionaries and grammars are what the act of speaking leaves behind. Needed for study, but not to be mistaken for the language itself. Diagram: L/LAB

Humboldt does not say dictionaries and grammars are useless — a scholar cannot work without them. But they are the sediment of the act of speaking, not the act. The part of a language “that must be regarded as dead” is not to be mistaken for the living language. A grammar is a trace of a language; as a map is not the terrain, it is not the map.

5. To Saussure and Chomsky

Humboldt (1836) ergon (product, deposit) / energeia (activity) Saussure (1916) langue (system) / parole (individual utterance) * energeia = "the power that produces parole out of the system" — it straddles both Chomsky (1965–66) competence (a generative rule system) / performance. * calls "competence" explicitly Humboldtian
"Language is not a thing but an act" underlies two of the twentieth century's founding distinctions. Diagram: L/LAB

In sum

Humboldt’s claimrestated
Language is not a product (ergon) but an activity (energeia)not a thing to point at, but an event that keeps being carried out
Language is “the ever-repeated labour of the mind”the work of making sound a vessel for thought; repeated each time you speak
Hence its definition must be geneticgrasp it by the procedure of production, not a list of parts
Dictionaries and grammars are a “dead deposit”scaffolding for study, but not the language itself
→ Saussure’s langue/parole, Chomsky’s competence“not a thing but an act” underlies the twentieth century’s basic distinctions

Why it is still read

The Humboldt feature: overview / this essay / inner form / worldview / infinite use of finite means / the organ of thought, and dialogue / typology and ranking.