25. Because of their minuteness.
If a great, nay, far the greatest part of the several ranks of bodies in the universe
escape our notice by their remoteness, there are others that are no less concealed from us by their minuteness.
These insensible corpuscles, being the active parts of matter, and the great instruments of nature, on which depend
not only all their secondary qualities, but also most of their natural operations, our want of precise distinct ideas of
their primary qualities keeps us in an incurable ignorance of what we desire to know about them. I doubt not but if
we could discover the figure, size, texture, and motion of the minute constituent parts of any two bodies, we
should know without trial several of their operations one upon another; as we do now the properties of a square or
a triangle. Did we know the mechanical affections of the particles of rhubarb, hemlock, opium, and a man, as a
watchmaker does those of a watch, whereby it performs its operations; and of a file, which by rubbing on them
will alter the figure of any of the wheels; we should be able to tell beforehand that rhubarb will purge, hemlock
kill, and opium make a man sleep: as well as a watchmaker can, that a little piece of paper laid on the balance will
keep the watch from going till it be removed; or that, some small part of it being rubbed by a file, the machine
would quite lose its motion, and the watch go no more. The dissolving of silver in aqua fortis, and gold in aqua
regia, and not vice versâ, would be then perhaps no more difficult to know than it is to a smith to understand why
the turning of one key will open a lock, and not the turning of another. But whilst we are destitute of senses acute
enough to discover the minute particles of bodies, and to give us ideas of their mechanical affections, we must be
content to be ignorant of their properties and ways of operation; nor can we be assured about them any further
than some few trials we make are able to reach. But whether they will succeed again another time, we cannot be
certain. This hinders our certain knowledge of universal truths concerning natural bodies: and our reason carries
us herein very little beyond particular matter of fact.