... for these truths hold good for everything that is, and not for some
special genus apart from others. And all men use them, because they are
true of being qua being ... For a principle which everyone must have
to understand anything that is, is not a hypothesis ... Evidently then,
such a principle is the most certain of all; which principle this is,
let us proceed to say. It is, that the same attribute cannot at the
same time belong and not belong to the subject in the same respect.
-- Aristotle
The agreement of so many kinds of animals in a certain common structure
which seems to be fundamental not only in their skeletons, but also
in the arrangement of the other parts - so that a wonderfully simple
typical form, by the shortening and lengthening of some parts, and
by the suppression and development of others, might be able to produce
an immense variety of species - allows a ray of hope, however faint,
to enter our minds, that here perhaps some result may be obtained,
by the application of the principle of the mechanism of nature (without
which there can be no natural science in general)....