What is the difference between 「慎重 」「用心深さ」and 「用心」?


I was originally looking up the word 慎重 and came across the other words. All
three of them are translated as "discretion." What's the subtle difference
and/or nuance among all three of them?




**用心** is mainly used as a suru-verb today, except in [this idiomatic
catchphrase](https://jisho.org/word/%E7%81%AB%E3%81%AE%E7%94%A8%E5%BF%83) and
用心深い. It refers to alertness against bad events, especially crimes.

**慎重** (na-adj) and **用心深い** (i-adj) are indeed similar in meaning, and
sometimes interchangeable. The former is simpler and used whenever you need
carefulness/alertness (like doing a math calculation or turning over a
pancake). The latter is usually more about security and used with risks such
as fire, thievery, fraud and natural disasters. 慎重な人 is a careful person who
takes time before trying something new, whereas 用心深い人 is a guarded/cautious
person who thinks they are at risk.

  * パンケーキを慎重に裏返す。
  * 試験の回答を慎重に確認する。
  * 窓を閉めたか用心深く見回る。
  * 彼に騙されないよう、話を用心深く聞いた。

**用心深さ** is the level/degree of 用心, just as **慎重さ** is the level/degree of 慎重.
(See [how to use さ](https://japanese.stackexchange.com/q/6665/5010))

