Apparent inconsistency in turning い-adjectives into nouns: 赤い→赤 vs 近い→近く

Question

Consider these adjectives:

• 赤い

• 近い

The い-declension works for sentence ending as well as directly modifying a noun:

Now if I want to express the adjective as a noun, for example I'm talking about "red(noun)" instead of "a red(adjective) thing":

• Remove い from 赤い to obtain the noun 赤.

Note that く can be appended to form 赤く which exists as an adverb.

赤になる and 赤くなる are both possible with similar meaning. This leads me to conclude that 赤 is independent from the 赤い which 赤く can be derived from.

But if I try the same thing for 近い I find that I have to add く for it to mean a noun.

• Step 1. Remove い from 近い to obtain the fragment(for lack of a better word) 近. 近 does not seem to be able to stand alone (or can it?).

• Step 2. Append く to 近 to obtain 近く₁

But 近く also exists as 近く₂ - the adverb that can be derived from 近い.

What is Step 2? It is different from the く-declension for adverbial usage.　Why doesn't 赤い get a 赤く noun that is derived from Step 2?

Answer 1

You are considering both of these words from the point of view of i-adjectives, but think differently.

赤 noun
→ 赤い/赤く adjective derived from a noun by attaching "-i/-ku"

近い/近く adjective
→ 近く noun derived from an adjective by zero-nominalization

Answer 2

There is often more than one way to turn adjectives into nouns. 赤さ and 近さ are nouns too.

The semantic relationship between 赤い and 赤 is quite different from the one between 近い and the noun 近く.

When learning about related words in different classes, I would learn the productive derivations (～さ is productive), and then deal with the fact that the rest have to be memorized. Sure, there are "sub-regularities" (赤→赤い, 青→青い, 白→白い) but you still have to memorize their limitations, since (following the example) there is no 緑い.

Maybe off topic, but for what it's worth: The artificial language Esperanto tried to make derivations like these completely regular and consistent, but in the language there are so many different derivational morphemes that many claim it's more bad than good.

Answer 3

The best way to think of this is that there are 2 types of words here.

Ones such as 近い that are adjectives being transformed into nouns and ones such as　赤 that are nouns being transformed into adjectives.

If you look at Japanese there are tons of words that are often used as nouns that can be made into adjectives just by adding い, for example 四角 -> 四角い、 黄色　-> 黄色い、 エロ -> エロい.

The groupings are also fairly patternistic. Colors are nouns, feelings are adjectives, shapes are nouns, distances are adjectives, etc.