In thinking about this question, I revisited a piece I wrote almost a decade ago , and found the questions and answers I struggled with then still meaningful today. I presented the issues in a dialogue between two imaginary characters — Bertie, who represented my rationalist side, and Ananda, my Buddhist side. Their dialogue went something like this:
Bertie: I think you’re being dishonest with yourself. I want to return to an earlier question as to whether what you’re espousing is really Buddhism or not. I think what you’re doing is adopting some Buddhist ideas about mindfulness, emptiness, impermanence, and non-self that happen to dovetail with Western ecological insights, quantum physics, cognitive science, phenomenology and existentialism. I think you’re melding them together, however, into a philosophy that’s no longer authentically Buddhist at its core. The aim of your philosophy seems fundamentally different from the aim of the Buddha’s philosophy: Your philosophy is about engagement with life and living one’s life in a philosophically justified way. The Buddha was interested in disengagement from life and the ending of rebirth, not about finding a meaningful life in the world. In many ways your belief system seems more Jewish than Buddhist: it celebrates and blesses creation and places the highest value on ethical life. In contrast, Buddhism seems disenchanted with creation and seeks annihilation.
Ananda: And I think you’re mistaken in thinking there’s this thing called “Pure Buddhism,” and that I’m adulterating it with Western ideas. There’s no such thing as Pure Buddhism. Buddhism has always been a philosophy in active dialogue with pre-existing cultures and philosophies. Tibetan Buddhism is the end product of a dialogue between Indian Mahāyāna and Tantra, Chinese Ch’an, and Pre-Buddhist Tibetan Bön beliefs and practices. Zen is a distillation of the dialogue between Indian Mahāyāna and Chinese Taoism filtered through the lens of Japanese culture. What could be more different than the spare aesthetic of Japanese Zen and the colorful profusion of Tibetan Vajrayāna? Which is more truly Buddhist? The Buddha’s original teachings, to the extent that we can discern what they were, arose as a protestant-like response to Brahmānic practice, and incorporated much of the pre-existing Vedic cosmology and prescientific understanding of the natural world into itself. Western Buddhism is just the latest version of an ongoing philosophical and historical dialogue, and it’s only natural that it be a dialogue with the Judeo-Christian tradition, as well as with Scientific Rationalism and Existentialism.
Bertie: Perhaps, but I still think you’re evading my main point: Your philosophy is a philosophy of full engagement with the world and Buddhism is a philosophy of withdrawal from the world. You’ve turned Buddhism on its head to suit your own purposes.
Ananda: I don’t think so. I think the real Buddhist message is mindfulness in each moment, non-identification with the perceptions, sensations, thoughts, cravings and aversions that arise in each moment, and the realization in each moment of the transience and interdependence of all phenomena. This mindfulness and insight leads to a life in which one no longer pursues self-aggrandizement, permanence, the accumulation of goods, or the pursuit of transitory pleasures; as a consequence, one lives compassionately and wisely in harmony with the world doing what each situation requires. It doesn’t require withdrawal from the world, but the release of one’s grasp on it. That’s Buddhism.
Many cultures of awakening derive from the Buddha’s teachings — diverging from them in some places, incorporating new elements in others – all of them deserving to be designated as a “Buddhism.” Existential Buddhism is one of many authentic branches on the Buddhist tree.
Ananda thinks so, too.
Bertie, on the other hand, still wonders.
-  Segall, S. (2003). "On Being a Non-Buddhist Buddhist." In Segall, S. (2003). Encountering Buddhism: Western Psychology and Buddhist Teachings. Albany: SUNY Press. ↩