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'.
The idea itself isn't bad.
The problem itself is complex.
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.
Standard modern spelling.
Rare kanji form using 其れ; mostly seen in older or literary texts.
Compound of それ (that) + 自体 (itself). The word 自体 is a Sino-Japanese word meaning 'itself' or 'in itself'.