  Locke’s great epistemological contribution to philosophy is a conception of human knowledge suitable for the experimental science of his day, one that in natural philosophy at least will replace the old, Aristotelian conception. According to the Aristotelian conception, scientific knowledge—scientia—is certain knowledge of necessary truths, which can in principle be expressed in syllogistic form, the conclusion following from self-evident premises. In the domain of natural philosophy, it is certain knowledge of real essences. Although Locke does not seriously entertain a radical skepticism—finding “a very manifest difference between dreaming of being in the Fire, and being actually in it” (E IV.ii.14, pp.537–538)—he recognizes that the demands of scientia are too stringent for the new, experimental science.  Nevertheless, the concept of scientia plays an important role as he develops his conception of the probabilistic sort of knowledge that is possible for humans in the domain of natural philosophy: he uses it as a foil when explaining why humans must settle for probabilistic knowledge there, scientia remaining always out of reach. Scientia can serve as that foil because it is attainable for human beings in certain domains; however Locke thinks that for nobler spirits, such as angels, it is also attainable within natural philosophy.