In the Encylopédie, d’Alembert hinted that “experimental physics,” unlike mathematical sciences, had a specific epistemic goal, which he characterized as “reducing” the multitude of phenomena to a few “principal facts” that should serve as principles for “deducing” all others. However, he did not detail precisely how he understood that goal, the status of the principal facts, nor the process of “multiplying” the phenomena. Neither did Diderot, who supported d’Alembert’s view. Herein, I present d’Alembert’s ideas and examine a prominent and highly successful case of experimental research as illustration: Charles Dufay, whose work d’Alembert certainly knew. Dufay not only managed to impose a firm order on the bewildering phenomena of electricity but also presented his research using the same epistemological notions that d’Alembert would himself later use. Similar notions may be found in the earlier writings of Edme Mariotte, who defined “principles of experience” as the goal of experimental research. This may suggest a specific tradition of spelling out the age-old idea of a double pathway of empirical science—from particular experience to general statements, and back again to the level of experiences—in a detailed manner that differed considerably from how Newton would frame it in his famous notions of analysis and synthesis.