For the curious and for beginners it is much easier to understand Zen by ascertaining what it is not. For example, Zen is not a science of behavior, which falls under psychology. Nor is Zen, when we look back to its historical roots, just about sitting. Zen does not embrace agnosticism, either, or materialism. Zen is not theological.
Zen, as with Buddhism, in general, is an enigma for the modern West. It could be argued that Zen has more in common with Plotinus (204/5–270 C.E.) and Meister Eckhart (1260–1328 C.E.) than with anything modern such as psychology or materialism.
Moving in the same direction as Zen, Plotinus and Eckhart point to a thoroughly transcendent state which is beyond the reach of the human sphere (in Buddhism the human sphere is prithagjana-bhumi) including the human body. Plotinus speaks of the One which transcends all manner of being and knowledge; while Eckhart speaks of the desert, the divine abyss, and the naught.
Such language, which is intended to point even beyond language, is hard for many Westerners to comprehend, who are habituated to sensuality and materialism as if there is nothing else besides. Why they bother with Zen is difficult to figure out (at least for me it is) given its mystical background. The only plausible reason is modern Zen is no longer the ‘mystical’ Zen of its homeland, China. Over time, it has been transmuted into a kind of minimalist religion in which experiencing the immediacy of the temporal body by means of ‘just sitting’ is the highest goal of Zen. Arguably, this is not the unique Buddhist meditation (jhâna/dhyâna) the Buddha described to Aggivessana in the Mahasacaka Sutra (M. i. 247) consisting of four meditations or dhyanas.