This paper aims to provide an account of Homberg's definition of chemistry as given in the first part of his "Essays de chimie" (1702). We show that his definition inverts the respective status traditionally assigned to chemistry and physics, which allows Homberg to establish chemistry as a science in its own right. For Homberg, chemistry alone is true, whereas physical explanations, though plausible, remain uncertain. Yet, in his chemical writings, Homberg did not hesitate to make use of physical principles in order to account for chemical phenomena. But the paradox is only apparent, because for Homberg, the use of physical principles amounted to providing an intelligible model for chemical phenomena, which would otherwise seem to be inconceivable (e.g. the transformation of mercury into gold). If such phenomena can be physically accounted for (by using figures and movements of corpuscles), we may be certain that they are real and not an experimental illusion. It is this view that implies the aforementioned inversion of status: whether physics is true is a moot question, because the only veritable science of matter is chemistry.