17. Supposition, that species are distinguished by their real essences, useless.
Concerning the real essences of
corporeal substances (to mention these only) there are, if I mistake not, two opinions. The one is of those who,
using the word essence for they know not what, suppose a certain number of those essences, according to which
all natural things are made, and wherein they do exactly every one of them partake, and so become of this or that
species. The other and more rational opinion is of those who look on all natural things to have a real, but
unknown, constitution of their insensible parts; from which flow those sensible qualities which serve us to
distinguish them one from another, according as we have occasion to rank them into sorts, under common
denominations. The former of these opinions, which supposes these essences as a certain number of forms or
moulds, wherein all natural things that exist are cast, and do equally partake, has, I imagine, very much perplexed
the knowledge of natural things. The frequent productions of monsters, in all the species of animals, and of
changelings, and other strange issues of human birth, carry with them difficulties, not possible to consist with this
hypothesis; since it is as impossible that two things partaking exactly of the same real essence should have
different properties, as that two figures partaking of the same real essence of a circle should have different
properties. But were there no other reason against it, yet the supposition of essences that cannot be known; and the
making of them, nevertheless, to be that which distinguishes the species of things, is so wholly useless and
unserviceable to any part of our knowledge, that that alone were sufficient to make us lay it by, and content
ourselves with such essences of the sorts or species of things as come within the reach of our knowledge: which,
when seriously considered, will be found, as I have said, to be nothing else but, those abstract complex ideas to
which we have annexed distinct general names.