Know Thyself in Chinese / Japanese...

不當和尚不知頭冷 is a Chinese proverb that literally translates as: [One who has] not been a monk [does] not know [the feeling of a] cold head.
I need to explain that a Chinese Buddhist monk always has a shaved head, and thus a cold head in winter.

Figuratively, this means: One cannot know the true meaning of hardship until one has experienced it oneself.

This is an idiom in Chinese, so the figurative meaning is what people perceive when they hear or read this phrase. Just as in English, when someone says, "The grass is always greener," one will think about the idea of jealousy, rather than the quality of one’s lawn.

This proverb is from Sun Tzu's (Sunzi's) Art of War.

It means that if you know and understand the enemy, you also know yourself. Four secondary characters come after this in the Art of War (not included here) which suggests you cannot lose a battle when you follow this philosophy.

In a very literal and somewhat-boring way, this can also be translated as “Estimate correctly one's strength as well as that of one's opponent.”

敵を知り己を知る is the Japanese version of “know your enemy, know yourself.”

There is a longer version of this proverb that adds, “...and you can win 100 battles.”

Note: Because this selection contains some special Japanese Hiragana characters, it should be written by a Japanese calligrapher.

知彼知己百戰不殆 is from Sun Tzu's (Sunzi's) Art of War. It means that if you know and understand the enemy, you also know yourself, and thus with this complete understanding, you cannot lose.

This proverb is often somewhat directly translated as “Know the enemy and know yourself, and you can fight a hundred battles without defeat.”

It can also be translated as “If you know both yourself and your enemy, you can come out of hundreds of battles without danger,” or “Know your enemy, know yourself, and your victory will not be threatened.”

敵を知り己を知れば百戦危うからず is the longer/full Japanese version of this proverb. This means “Know your enemy, know yourself, and you will not fear a hundred battles.”

Others will translate this as “Know thy enemy, know thyself, yields victory in one hundred battles.”

Note: Because this selection contains some special Japanese Hiragana characters, it should be written by a Japanese calligrapher.

不當和尚不知齋戒苦 literally translates as:
[One who has] not been a monk [does not] know the suffering of [being on a] vegetarian diet.

This is a bit like the “walk a mile in another man's shoes” saying. Basically, it's about you cannot fully understand the plight of others until you experience it yourself.

不眠之夜长久交知人心 literally translates as: [Only one who does] not sleep, learns how long the night is; [Only by] long acquaintance [does one] learn a person ['s true] character.

Basically, this proverb suggests that we really need to experience something intimately and for a long time to really know everything about it.

This can also be translated as “Spending years with someone is the only way to know them.”

Note: Sometimes this proverb is split into just the first or second idea alone (first 5 or last 5 characters only).

知人者智自知者明 is a Chinese proverb that means “Those who understand others are clever, but those who know themselves are truly wise.”

This proverb is derived from the teachings in Laozi's (Lao Tzu's) 道德經 or Daodejing.