  Du Châtelet believes that hypotheses are necessary because not all phenomena can be explained through reliance upon first principles alone—there is a gap between first principles and observed phenomena in the world in the sense that the scientist cannot deduce the cause of those phenomena directly or through chains of deduction from the first principles. Neither can experiment directly tease out such a cause. “Hypotheses are… sometimes very necessary… in all cases when we cannot discover the true reason for a phenomenon and the attendant circumstances, neither a priori, by means of truths [identified as principles in §53] that we already know, nor a posteriori, with the help of experiments” (IP §60). And: “[P]hilosophers frame hypotheses to explain the phenomena, the cause of which cannot be discovered either by experiment or by demonstration” (IP §56).