形容詞 (adjectives) can be made into nouns by swapping the end い with さ.
- 自然は美しい -> Nature is beautiful
- 自然の美しさ -> The beauty of nature
- 今日は暑い -> Today is hot
- 今日の暑さは格別だ -> Today’s hotness is at another level
- このゲームは面白い -> This game is fun
- このゲームの面白さが解らない -> I don’t understand what is fun about this game
Things are more complicated for 形容動詞. Some of 形容動詞 are already nouns as they are as well, some can be made into nouns by adding さ and some cannot be made into nouns at all.
- この仕事は大変だ -> This job is very difficult
- この仕事の大変さはやってみないと分からない -> You won’t understand how difficult this job is unless you do it yourself
- 健康は何よりも大事だ -> Health is more important than anything
- 健康の大事さが身に沁みた -> I felt the importance of health to my core
- この機械は操作が簡単だ
- この機械は操作の簡単さが客に好評だ -> The ease of the use of this machine
Adding the suffix 性 also nominalize 形容動詞, even though it is for literary and/or technical use. 形容動詞 that end with 的 are usually nominalized this way.
- 合理的な考え方 -> Rational thinking
- 合理性に欠ける考え -> An idea that lacks rationality
- 彼は銃を合法的に入手した -> He obtained the gun legally
- 安楽死の合法性に関する議論 -> Debate over the legality of euthanasia
Some 形容動詞 can be nominalized with either さ or 性, but not all. There is no hard and fast rule, and it is rather idiomatic.
- 重要 (important) -> 重要さ or 重要性 (importance)
- 敏捷 (agile) -> 敏捷さ or 敏捷性 (agility)
- 可能 (possible) -> 可能性 (possibility) but never 可能さ
- 簡単 (simple, easy) -> 簡単さ (simplicity, ease) but never 簡単性