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The Aristotelian scholastic approach to knowledge saw scientific knowledge as certain knowledge of necessary truths, with conclusions deduced from premises that were self-evident. Like many others of his times, Locke did not believe that this sort of knowledge was generally possible in natural philosophy. He sought to replace these stringent demands with ones more compatible with the new experimental science, such as that practised practiced by the Royal Society. He took knowledge to be "nothing but the perception of the connection and agreement, or disagreement and incompatibility, of any of our ideas", with our ideas derived ultimately from sensations. [[CiteRef::Locke (2015d)|p. 196]][[CiteRef::Kochiras (2014)]] Locke distinguished between two sorts of knowledge, knowledge of '''nominal essences''' which are the set of observable qualities we use to classify a thing, and knowledge of '''real essences''' which are the causal grounds of a substance's perceivable qualities. For Locke, the deepest sort of knowledge one might have in natural philosophy would be knowledge of the real essences of material substances and the necessary connections to qualities flowing from them. Locke imagined this to be knowledge of the corpuscles that make up matter and their sizes, shapes, and arrangements. Given such fundamental knowledge, we could deduce the tertiary qualities of substances; their powers to produce certain effects in other substances. Just as a locksmith knows that a particular key opens one lock but not another, we could know that opium produces sleep, and hemlock causes death and the reasons why. [[CiteRef::Locke (2015d)|p. 212]][[CiteRef::Kochiras (2014)]] But Locke supposed that such knowledge was, for the most part, beyond human faculties because corpuscles are too small to be discerned by human senses. He wrote that "But while we lack senses acute enough to discover the minute particles of bodies and to give us ideas of their fine structure, we must be content to be ignorant of their properties and ways of operation, being assured only of what we can learn from a few experiments. And what we can learn for sure in that way is limited indeed." [[CiteRef::Locke (2015d)|p. 212]] In making this case, Locke nonetheless felt confident in relying on the corpuscular hypothesis "because that’s the theory that is thought to go furthest in intelligibly explaining those qualities of bodies; and I fear that the human understanding hasn’t the power to replace it..." [[CiteRef::Locke (2015d)|p. 208]] While knowledge of real essences, was, for the most part, inaccessible to humans, he supposed that certain other epistemic agents, such as God, the angels, and the inhabitants of other planets that might have different and more acute senses, could be capable of such knowledge. [[CiteRef::Kochiras (2014)]][[CiteRef::Locke (2015d)|p. 211]]