Although the modern day religious trappings of both Zen Buddhism and Chinese Taoism would seem to place the traditions in completely different lineages, at their core both schools are related. Fundamentally, both Zen and Taoism are related in their core philosophies. Both schools function as philosophical systems that can be used to explore the basic nature of reality. The relation between the two is their insistence on stepping away from the categorizing properties of the rational mind to encounter the state of the world as experienced through intuitive knowledge.
Taoism, for at least the past 2000 years, seems to have concerned itself most visibly with methods of health and wellbeing, and with tools, such as the I Ching, for the divination of future life courses. Zen, on the other hand, has come to be known as a school of discipline for the cultivation of mental wellbeing. At first glance, both schools seem to be religions, that is, traditions based on belief rather than investigative practice. Both schools have collected over time the trappings of ceremony and worship, for better or worse. This outward decoration actually serves to hide the related philosophical approach both schools have to the eternal problem 'What is reality?'.
In his Shobogenzo, the ancient Zen master Dogen lays out not only his belief in 'only zazen', but also his understanding of the differences in the operation of the human mind. Dogen's system of thinking, not-thinking, and non-thinking differentiates between the mental behaviours that operate as one meditates and as one lives. For Dogen, the cultivation of zazen lay in training the mind past the states of thinking and not-thinking, both of which imply a mind with an agenda, to the state of non-thinking. Non-thinking is the state that always underlies mental activity, whether that activity is active consideration of mental images, thinking, or the active pushing away of mental activity.
Non-thinking is the pure state of no mental agenda, a state of mental silence and peace synonymous with Buddha nature. Dogen's authentication is the practice of residing in non-thinking, once all mental agenda has been dropped. This represents a move away from the inherently agenda-laden state of the thinking and not-thinking mind, to the state of pure awareness that is non-thinking. If this state is cultivated and authenticated for a long enough period of time, one recognizes the pure state of reality that non-thinking not only represents, but allows to be seen clearly. In this state, intuitive thinking and living can arise in the mind without being hindered by compulsive mental thought.
The Chinese Taoist sage of the 4th century BC, Zhuangzi, never differentiated between the various states of mental activity as Dogen did. Zhuangzi, for the most part, did not concern himself with meditative practice. Though the Taoist work that is named after him seems to concern itself largely with the relative nature of existence, at its core it is a guidebook on how to live life in an intuitive manner outside of the confinement of the judging mind. In other words, the Zhuangzi of Zhuangzi is an attack on the rational mind, and its study shows the way to break down and shock the rational mind into a state synonymous with Dogen's non-thinking, or Buddha nature.
This can clearly be seen in the opening chapters in which the rational mind is shown to work on systems of relative categorization that are useless when it comes to knowing, and living in harmony with, pure reality. Zhuangzi's story of 'The Dexterous Butcher' illustrates the expertise and skill that can be experienced when one gets out of one's own way by switching off the rational mind. Butcher Ting moves with experience and skill because he carves meat in a state of non-thinking. Elsewhere in the Zhuangzi, the reader is encouraged to cultivate a mind like a mirror; seeing and reflecting all, retaining nothing. In this way Zhuangzi encourages us to abandon the categorizing function of the rational mind, and to live in the state of intuitive non-thinking, promising the result to be harmony and skill.
As can be seen, Zen and Taoism are related through a synonymous philosophical core. Dogen's practice of Zen cultivation-authentication, and his thinking, not-thinking, and non-thinking system reflect practice that is synonymous with the philosophical approach Zhuangzi encouraged. Both men taught that to encounter pure reality, one must move past the mind that splits the world into categories, into a state of non-thinking. In this final state, both agreed, the nature of pure reality could be known.