  One cornerstone of Gassendi's anti-Aristotelianism is the suggestion that there is nothing necessary about the way the world is.  God, he proposes, could have made the world work in any number of ways, and the contingent history and character of Creation means that there is nothing immutable about the essence of a material thing. (That a ‘substance’, in either the Aristotelian or Cartesian sense, might have an immutable essence, is a different matter, and insofar as Gassendi has such a notion (for example, with respect to space, time, matter, and void) he agrees that such things feature unchangeable sine qua non characteristics.) Moreover, Gassendi maintains, regardless of whether there are any essences and whether they might be mutable, there are none to which we have any epistemic access. The sole originating source of our knowledge is the information the senses provide, such that what we know is closely linked to what we can perceive. However, as Descartes notes, we can perceive only appearances. Gassendi draws from this point the very uncartesian lesson that appearances are all we can know about, too—thereby ruling out knowledge of unperceivable essences. One line of this reasoning can be found in his discussion of classical skeptical tropes concerning the relativity of evidence from the senses to individual experience—that honey tastes sweet to me, though bitter to you; and that fire seems hot to us, though not so to insects that live near fire (O III (DM) 388b; R 535). Since different people have distinct experiences, our knowledge of honey's taste or fire's heat differs from person to person and thus is not a reliable guide to invariable characteristics of, for example, the honey or fire. In cases like these we know a thing's qualities only as we record them on a subjective basis. Such sensory information, based on experiences which vary intersubjectively, cannot yield judgments about a thing's qualities which do not vary in that (or any other) way. Hence we lack knowledge of the thing's essence, if indeed there is one. More broadly, from our principal source of ideas—the senses—we know only how things appear to us (O III (DM) 311b-312a; R 184). (If we are to have knowledge of an object's essence, Gassendi proposes, such requires a “perfect interior examination” of that object, which is apparently not something we may gain from empirical study.)