We intrinsically have the countenance of the person of suchness and so need not be anxious about the essence of suchness. Because anxiety is itself the essence of suchness, it is not anxiety. Moreover, we need not be startled by the essence of suchness being this way. Even if suchness appears startling and suspicious, it is suchness all the same.

—Master Dogen1

Suchness is a poor translation of the Japanese "nyo ze,"2 which literally means "it is like."3 It has also been called "thusness" and "ainsité" (noun form of French for "in this way" or "so"; a bit more accurate translation).4 The phrase is a Buddhist term used in the Lotus Sutra (2:24) to outline the phenomenen of existence: it is said to be "like" ten aspects including appearance, power, influence and consistency.3 Following what seems to be a tradition of elusive definitions of Buddhist terms, it is difficult (some would say necessarily impossible) to describe what suchness is in writing, or indeed using any words or language at all. Instead of trying to pin down its wiley definition, I'll explain a saying.

In the beginning mountains are mountains, rivers are rivers. After practice for some time, mountains are not mountains, rivers are not rivers. After that going further, mountains are again mountains, rivers are again rivers.

The study of Zen is often summed up with this strange couple of sentences. Most people live their entire lives at the "beginning," when things are as they seem. After studying Zen for a while, however, nothing makes sense any more — words and labels all become illogical and pointless, mountains are the same as rivers which are the same as nothing and everything, and there is no reason to do anything or nothing. This is the Zen "emptiness." However, when one pursues the study, one punches through this "emptiness" and realizes that mountains are still mountains, rivers are still rivers, words and labels are indeed just words and labels, and you still eat when you're hungry and sleep when you're tired. This is "suchness."

Your true nature is something never lost to you even in moments of delusion, nor is it gained at the moment of Enlightenment. It is the Nature of the Suchness. In it is neither delusion nor right understanding.

The Zen Teachings of Huang Po (trans. by John Blofeld)5

Trying to classify "suchness" as a person/place/thing/idea is a bit frustrating. Suchness is at once an idea, a state of being, a place in one's mind that is the entire mind, and, once the previous is realized, one becomes a person of suchness.

personal knowledge
1: Anxiety: Startling Suchness: http://www.mro.org/zmm/talks/myotai20.htm
2: Buddhist Myths, Parables, and Essays: http://www.gakkaionline.net/Myths/Suchness.html
3: The True Entity of All Phenomenon Is the Wisdom to Grasp the Truth of Life: http://www.sgi-usa.org/buddhism/library/SokaGakkai/Study/LectLS/Lectur13.htm
4: Song of Precious Mirror Samadhi: http://www.gileht.com/Chan/The_Song_of_the_Jeweled_Mirror_Samadhi_08_for_html.htm
5: Mystic Cowboy: The nature of suchness: http://mysticcowboy.org/archives/000518.html