He argues that science cannot be derived merely from the findings of sense experience, but is also intellectually constructed through the imposition of a set of a priori conventions on those findings. Bergson's philosophy enjoyed a vogue in France after the First World War comparable to that of Sartre's existentialism after the second. Bergson is neglected at the time of writing, even though his philosophy has the rare merit of taking seriously both science and the subjective reality of lived experience. Paradigmatically, phenomenology is the pre-suppositionless description of the contents of experience, without any prior ontological commitment to the objective reality or causal properties of those contents. By 'knowledge' is meant here 'all knowledge' and so, a fortiori , 'all philosophical and scientific knowledge'.
Existentialism is a reaction against both metaphysics and the essentialism of 'pure' phenomenology.
Its principal thesis is that existence is logically prior to essence and that human essence is not determined a priori but freely created by human actions. Sartre's 'existential' phenomenology is expounded not only in philosophical works but in plays, novels, short stories, and political tracts. The most brilliant existentialist writer was Simone de Beauvoir. Whether Sartre's putative synthesis is successful or not, in this effort modern French philosophy was engaged in trying to solve genuine metaphysical problems. The most influential French philosopher at the time of writing is Jacques Derrida.