Here Locke is following the epistemology associated with the Royal Society. The
founding members of the Royal Society rejected the investigation of real essences as too
ambitious, and thought that science should confine itself to the systematic description and
prediction of observable and quantifiable qualities (see van Leeuwen 1963, pp. 40–41). The
idea that this should be one of the goals of science is a real advance that puts its finger on a
problem with the original Aristotelian understanding of scientific explanation. According
to this understanding, one explains why a thing acts as it does by discovering its real
essence, and showing how its causal activity flows from this real essence; and the question
of why it has that essence is not one that needs an answer, because all there is to being that
thing is its being a thing of that essential kind.