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.

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.

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.”

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.

This is referred to as passage or chapter 33 of the Dao De Jing (often Romanized as “Tao Te Ching”).

These are the words of the philosopher Laozi (Lao Tzu).

Notes:

During our research, the Chinese characters shown here are probably the most accurate to the original text of Laozi. These were taken for the most part from the Mawangdui 1973 and Guodan 1993 manuscripts which pre-date other Daodejing texts by about 1000 years.

Grammar was a little different in Laozi’s time. So you should consider this to be the ancient Chinese version. Some have modernized this passage by adding, removing, or swapping articles and changing the grammar (we felt the oldest and most original version would be more desirable). You may find other versions printed in books or online - sometimes these modern texts are simply used to explain to Chinese people what the original text really means.

This language issue can be compared in English by thinking how the King James (known as the Authorized version in Great Britain) Bible from 1611 was written, and comparing it to modern English. Now imagine that the Daodejing was probably written around 403 BCE (2000 years before the King James Version of the Bible). To a Chinese person, the original Daodejing reads like text that is 3 times more detached compared to Shakespeare’s English is to our modern-day speech.

Extended notes:

While on this Biblical text comparison, it should be noted, that just like the Bible, all the original texts of the Daodejing were lost or destroyed long ago. Just as with the scripture used to create the Bible, various manuscripts exist, many with variations or copyist errors. Just as the earliest New Testament scripture (incomplete) is from 170 years after Christ, the earliest Daodejing manuscript (incomplete) is from 100-200 years after the death of Laozi.

The reason that the originals were lost probably has a lot to do with the first Qin Emperor. Upon taking power and unifying China, he ordered the burning and destruction of all books (scrolls/rolls) except those pertaining to Chinese medicine and a few other subjects. The surviving Daodejing manuscripts were either hidden on purpose or simply forgotten about. Some were not unearthed until as late as 1993.

We compared a lot of research by various archeologists and historians before deciding on this as the most accurate and correct version. But one must allow that it may not be perfect, or the actual and original as from the hand of Laozi himself.

This is the Mawangdui version of Daodejing chapter 54.

空無 is “nothingness” in a Buddhist context.

The first character means empty but can also mean air or sky (air and sky have no form).

The second character means have not, no, none, not, or to lack.

Together these characters reinforce each other into a word that means “absolute nothingness.”

I know this is a term used in Buddhism, but I have not yet figured out the context in which it is used. I suppose it can be the fact that Buddhists believe that the world is a non-real illusion, or perhaps it's about visualizing yourself as “nothing” and therefore leaving behind your desire and worldliness.
Buddhist concepts and titles often have this element of ambiguity or, rather, “mystery.” Therefore, such ideas can have different meanings to different people, and that's okay. If you don't get it right in this lifetime, there will be plenty more lifetimes to master it (whatever “it” is, and if “it” really exists at all).

Soothill defines this as “Unreality, or immateriality, of things, which is defined as nothing existing of independent or self-contained nature.”