Among Buddhist traditions, Zen stands out as the synthesis of the best aspects of the two most important early traditions: Theravada and Madhyamaka Buddhism. Among the most important aspects of Theravada Buddhism is its emphasis on meditation. A clear expression of this comes in Buddhaghosa's work entitled the Visuddhimagga (The Path of Purity). The personal ideal for the Theravada tradition was the arahat, a monk, who achieves enlightenment, in part through a dedicated practice of meditation. Among the most important aspects of Madyamaka Buddhism is the concept of sunyata (emptiness). Sunyata is a conclusion drawn about the world that rejects all metaphysical views, particularly the existence of permanent entities and the non-existence of permanent entities. By coming to understand this view of the world, the Madyamika becomes poised to escape the ensnaring attachments to the world. Meditation is, of course, an important aspect of Madhyamaka practice and the notion of sunyata is not completely absent from early Buddhist traditions, but no tradition brings them together so deliberately as Zen Buddhism.
The synthesis of these ideas is made clear in the writings of the thirteenth century monk, Dogen. According to Dogen, zazen or sitting meditation, is "the authentic gate to free yourself." When sitting in meditation for long periods of time, one is easily brought around to focus attention on the spatial and temporal present and acquire an intimate or immediate relationship with the world. For Dogen, such a relationship is at the heart of the Zen approach to enlightenment. Equipped with the understanding of sunyata, that all things exist dependently (each dependent on the other) and that the conventional understanding of existence is illusory, the person meditating obtains "wisdom beyond wisdom." The illusion of the duality of the self and other, the subject and object, is revealed.
In The Essential Dogen, Tanahashi and Levitt bring together an engaging collection of Dogen's writings. The passages are short, seldom more than a single paragraph and sometimes only sentence of two. They are often enigmatic, though for someone with a background in the Mahayana tradition, the central ideas slowly surface. It is both a virtue and a failing of the work that Dogen's words are left to speak for themselves. Without interpolated commentary, one comes away with a direct appreciation for Dogen as a writer; however, the uninitiated would greatly benefit from occasional commentary. Levitt does provide a brief introductory essay to the work, but he touches on only a few ideas. Personally, I would direct the reader to essays by D.T. Suzuki before picking up The Essential Dogen.