noun
in itself; the thing itself; per se
Used to emphasize that something is being considered on its own, apart from external factors. Often follows a noun or pronoun to mean 'that very thing' or 'in and of itself'.
そのアイデアそれ自体は悪くない。
The idea itself isn't bad.
問題はそれ自体が複雑だ。
The problem itself is complex.
自体 is a suffix meaning '-itself' and attaches to nouns (e.g., 問題自体). それ自体 is a fixed phrase meaning 'that itself' or 'in itself', often used as a standalone adverbial phrase.
そのもの also means 'the very thing' or 'itself', but is more emphatic and often used after a noun to stress identity (e.g., 人生そのもの). それ自体 is more neutral and can be used adverbially.
Compound of それ (that) + 自体 (itself). The word 自体 is a Sino-Japanese word meaning 'itself' or 'in itself'.