Existentialism:

Those who adopt an existential posture typically express the conviction that human beings can never be certain of anything beyond the sheer fact of their own existence. What human beings assert that they know is wholly capricious and unreliable. The subjective quality of the process of knowing is seen by the existentialist as inevitable. When we claim to “know” something, the “knowing” seems to be rather an intuitive grasp that has, or can be given, personal significance. There is nothing stable or objective about it. It is neither rational nor empirical  in the traditional sense. To state that one knows is merely to express a personal affirmation that may shift or change at random with no outside criteria by which it may be evaluated or criticised. It boils down to this: Truth is what I affirm it to be. It is for this reason that there can be no existential theory or system of of knowledge. It is for this reason that non-existentialists are likely to apply, and existentialists not likely to reject, the label “irrationalists.”

Atheistic existentialists seem to be completely disillusioned about the possibility of knowing or finding “truth.” a person may respond or act, but in no way can that person be certain about anything. These existentialists tend to deny that life has, or can have, any more than a subjectively chosen meaning.

Religious existentialists also exhibit extreme pessimism about the acquisition of reliable knowledge or fundamental truth. They are likely to argue that human beings cannot know; they can simply believe. And belief is a non-rational “leap of faith.” A person affirms himself or herself  by a jump into darkness. There is no way to know  for sure where or why. The only justification for such a leap is “This I am.”

Zen Buddhism:

Zen Buddhism is one of the Eastern philosophies that put little store in intellectual theories of epistemology. Although it is risky to use Western language to characterize an Eastern philosophical posture, the Zen way of knowing appears to favour the intuitive and the mystical elements in the process. The goal of Zen Buddhism is to achieve an enlightenment (satori) that transcends the formal categories of reasoning as well as the generalizations resulting from the amassing of discrete experiences in the sensory world. Any system of formal or objective “knowing” is devalued on the ground that it provides only an abstract picture of total reality and thus distorts rather than aids comprehension. The futility of trying to attain precise and structured knowledge is illustrated by the case of a man who paints lines on his plate glass window in order to locate more accurately specific points of interest in the vast panorama outside. The more coordinates he paints on the window, the more the panorama is deformed and segmented, and the less he or anyone else can see of the magnificent view.

For the followers of Zen, total reality simply exists. “Knowledge about” the reality separates one from direct “acquaintance with” all-inclusive “being.” The human experiential vision should be total and peripheral rather than partial and narrowly focused. The Western compulsion to structure and divide knowledge is viewed as a misguided attempt to describe the indescribable — to substitute a wall and a door for the “gate-less gate.” In the world-view of Zen, the wise person, or sage, is one who ceases to make objective distinctions and becomes identified with the underlying oneness of that which is. Such as antiknowing  posture is likely to be seen by Western philosophers as a type of skepticism.

Critique of Existentialism and Zen:

The rejection found in existentialism and Zen of all ways of knowing is too subtle to make sense to the average person. Human beings continue to ask what they assume to be answerable questions. The posture of antiknowing is fatalistic, defeatist, and unreasonable. The avid pursuit of systematic knowledge has brought about demonstrable and desirable results. Humankind had to seek, and organize, and build knowledge to raise itself from the level of bestiality. Precise, consistent, and communicable knowledge has promoted human mastery of the environment, raised standards of living, and enriched experience. Without an active and ordering intellect, a human being would simply not amount to anything. Antiknowing thus becomes a barrier to human evolution.

Source: INVITATION TO PHILOSOPHY: ISSUES AND OPTIONS