Buddhism’s fundamentals are altogether called the Dharma. The Four Noble Truths are included, as well as the Middle Way, or moderation in everything. The meaning of Nirvana is given as nothingness, in the sense that the mind is neither positive or negative, but in a perfect state of neutrality. Because Buddhism crossed into van Gogh’s life through Japonisme, the school of Buddhism he followed is Japanese as well. Japan has two major sects of Zen Buddhism, Rinzai and Soto, both coming from India through China. Rinzai involves looking at a sand garden for long periods of time and training for various kinds of combat, including archery, karate, and kendo. Discipline and physical conditioning along with koan, or irrational dialogue and questioning to intellectually shock the mind into awareness, are practiced to reach Nirvana in a sudden realization of awareness. On the other hand, Soto Zen consists mainly of sitting meditiation, or zazen, in order to reach enlightenment.
It seems that van Gogh followed the Rinzai school of Zen, since there is no evidence that he did zazen at all. Although he also did not practice any combat training, his physical stress may be instead from his painting in Arles. With great discipline he forced himself to focus on strictly painting, and perhaps hoped to reach enlightenment in this way.
The method of koan would then explain van Gogh’s mental instability, because this involves such questions like, “Two hands clap and make a sound, what is the sound made by one hand clapping?” Van Gogh committed many illogical actions, such as attempting to kill Gauguin and cutting off his own ear. This may in fact be his own sense of sudden realization, because he swings drastically between violent impulses and remorseful regret, and back again.