Why are adjectives declined in German?

Question

This is a rather specific question not of language syntax but rather of its origin/history. I’m just trying to understand the specific reasons behind the need for adjective declension.

I have read somewhere that, it is done to indicate the case and differentiate singular vs plural.

However I'm not sure if this holds true. Let me explain with a few examples.

• Nominative:

  Die blaue Hose ist groß. (Singular)

  Die blauen Hosen sind groß. (Plural)

  Now, as we can seen blau is declined in this case differently for singular and plural. The distinction in singular and plural is due to the noun’s ending as we can see (Hose vs. Hosen), and since the verb sein is used, we know that it is nominative. So why even bother such declension?

• Similarly, in the accusative case:

  Ich habe eine blaue Hose. (Singular)

  Ich habe die zwei blauen Hosen. (Plural)

  Here also, we know haben is an accusative verb, so case is indicated through it and the noun differentiates singular and plural. So why do we need to do it?

In nominative and accusative cases, there is no consistency of declension while in dative case there is mostly a common -en ending.

Can someone explain to me why exactly we do adjective declension? Is there any specific role behind it or is it just an accepted norm?

I’m not being judgemental or anything; I am just curious, in particular because it is so different in English (which has the same origin as German).

Answer 1

Inflection only superficially marks cases and numbers. What it really marks is roles of entities in a proposition.

Having different forms for accusative and nominative allows speaker to distinguish between subjects and objects, and hence between actors and recipients, on the basis of individual word forms rather than relative order among words. This is why German sentences can be reordered relatively easily to convey nuance, focus etc.

Not surprisingly, across the world richer inflection co-occurs with freeer word order - classical Latin is near one end of the spectrum, and isolating languages like Mandarin near the other. But the important constant is that all languages are roughly equipotent at getting the relevant distinctions across, but they use different means to do it. Inflected forms cater to one form of human memory (semantic), order of words to another (episodic memory). Therefore, there's usually no good answer to the question "Why does language A have feature X?" until you consider all features of A in combination.

Answer 2

Since there are languages without adjective inflection, there can be no "need" for it. English survived the loss of inflection just fine:

(Old English)

mid welwillendum mōde (dative, mit wohlwollendem Gefühl)
fela untrumra manna (genitive, literally "viele kranker Männer")

As to why English differs from its Germanic brethren, I blame the French.

You make the following claim:

Here also, we know haben is an accusative verb, so case is indicated through it and the noun differentiates singular and plural

But it's actually the other way around: We only know that haben governs the accusative because there are case markings.

Ich brauche grobes Mehl/frische Ananas. – Hier ist grobes Mehl/frische Ananas.
Ich brauche schwarzen Pfeffer. – Hier ist schwarzer Pfeffer.

I find it quite dubious to assign a purpose to grammatical structures. For instance, people will say that case marking makes word order more liberal, and that is true to some extent. But note that German word order is more liberal than English word order, even when case is not marked clearly:

Die Mutter sieht die Tocher nur noch selten.

This sentence is ambiguous: Both mother and daughter can be interpreted as the subject.

Compared to English, which has lost its adjective endings, German allows much larger adjectival phrases, and adjective endings might help in processing such phrases. Compare the simple noun phrase

der erfahrene Fachmann

with the seemingly superfluous marking on the adjective to a more complex example like

der in solchen Fragen erfahrene, mit allen Wassern gewaschene Fachmann

where the adjective ending signals the closing of an adjectival phrase and tells the listener to wait for a masculine singular (and not dative feminine or plural) noun. I don't know whether I would call it a purpose, but it's a connection at least.