Linchi, founder of the Rinzai school of Zen Buddhism (Rinzai is the Japanese form of Linchi's name). Linchi is considered by many to be the greatest influence on Zen next to Bodhidharma. He was a monk in 9th century China.
Linchi is as well known for his unpredicatable teaching methods as for his philosophy. He was known for shouting and striking at his students to enlighten them. He was also known for insulting sacred Buddhist truths, declaring "If you meet the Buddha on the road, kill him" and "Nirvana is a piece of toilet paper" (or whatever passed for toilet paper in 9th century China).
In contrast to his over the top rhetoric, his philsophy itself was moderate and even. One of the lessons of his that I personally learned the most from was that trying to deny yourself or exert yourself too much are both detrimental to realization. Another basis of his teachings is that a person can be enlightened immediatly without needing a lifetime of meditation, or eons of learning in the Six Worlds.
That being said, I won't attempt to explain the thinking of one of the greatest minds of Buddhism.