People suffering from Existential Angst are either not convinced that they exist, unsure why they exist or not at all convinced that anything really exists at all. The Cartesian summary of this hypothesis (or really a lack of any clear hypothesism) is "I (if there is an I) might think, therefore I (ditto) might (if anyone actually cares) be". Some take is as far as "There's no point in even thinking as I might not be anyway". Most French employers allow paid leave to be taken for these reasons.
Until the invention of both Creationism and the Theory of Evolution, humanity naturally suffered hugely from the complete lack of knowledge of why they were here, what they were meant to be doing and whether any endeavours they undertook in their lives would, in the great scheme of things, be worth anything.
Despite the advent of modern philosophical techniques which quite clearly demonstrate humanity's proud and noble purpose upon the earth some slack-minded FOOLS still persist in whining on and on in a most tiresome manner about how, to a greater or lesser extent, they feel that there might not be any particular point to our existence beyond the basic animal obsessions of genetic perpetuation for its own sake. The definition of the Earth and its contents are accepted as "real" on a purely physical level but not as having any philosophical import.
Caco Ergo Sum
Like other unfortunate side effects of modern philosophy, voiding one's bowels helps ground the sufferer by causing all senses to work in harmony and gives tangible proof of one's existence.