There is a huge number of laments concerning the lack of intelligibility of quantum mechanics. Some ingredients of quantum mechanics may however be possibly understood by referring to first principles, that is to say to basic principles (or postulates) which are clear and distinct to the intuition. In particular, if we rely on a first principle called non-singularity principle, which may be viewed as a hypothesis, we claim that quantum mechanics can be viewed as the a priori consequence of a rational demand. The status of the non-singularity principle, obvious to most physicists, may however be criticized, on the basis that there is no universal intuition and that any statement is in principle revisable.