Translation guide
The English word "itself" is used for emphasis or to refer back to a thing. Japanese often omits the equivalent word, uses specific structures for emphasis, or relies on context. Direct translation is rarely natural.
The idea itself isn't bad.
そのもの (そのもの) attached to a noun also means 'itself', often with a nuance of 'the very thing' or 'in and of itself'.
人生そのものが旅だ。
Life itself is a journey.
In many cases, Japanese simply omits the equivalent of 'itself' because the context makes the emphasis clear. Adding 自体 or そのもの can sound overly emphatic.
この機械は自動で動く。
This machine operates by itself.
To indicate that something does something without help or is alone.
Used for inanimate objects or things that happen spontaneously, meaning 'by itself' or 'automatically'.
ドアがひとりでに開いた。
The door opened by itself.
Means 'automatically', often used for machines or systems.
この機械は自動で動く。
This machine operates by itself.
Means 'independently' or 'alone', more formal or technical.
その装置は単独で機能する。
The device functions by itself.
To say that something has a certain quality considered on its own, without external factors.
それ自体 (それじたい) means 'in itself' or 'per se', used to consider something independently.
お金それ自体は悪くない。
Money in itself is not bad.
As above, そのもの can also convey 'in itself'.
存在そのものが奇跡だ。
Existence itself is a miracle.
English often uses 'itself' for emphasis where Japanese uses context or a different structure. Directly translating 'itself' as 自分 (じぶん) is incorrect for things; 自分 is for people.
If the sentence is clear without 自体 or そのもの, it's often more natural to leave it out. Overusing these can sound like you're translating from English.