which we perceive heat and cold, not by the sense of touch._ VII. _Of
the sense of extension, the whole of the locomotive muscles may be
considered as one organ of sense._ VIII. _Of the senses of hunger,
thirst, want of fresh air, suckling children, and lust._ IX. _Of many
other organs of sense belonging to the glands. Of painful sensations
from the excess of light, pressure, heat, itching, caustics, and
electricity._

I. Philosophers have been much perplexed to understand, in what manner we
become acquainted with the external world; insomuch that Dr. Berkly even
doubted its existence, from having observed (as he thought) that none of
our ideas resemble their correspondent objects. Mr. Hume asserts, that our
belief depends on the greater distinctness or energy of our ideas from
perception; and Mr. Reid has lately contended, that our belief of external
objects is an innate principle necessarily joined with our perceptions.

So true is the observation of the famous Malbranch, "that our senses are
not given us to discover the essences of things, but to acquaint us with
the means of preserving our existence," (L. I. ch. v.) a melancholy
reflection to philosophers!

Some philosophers have divided all created beings into material and
immaterial: the former including all that part of being, which obeys the
mechanic laws of action and reaction, but which can begin no motion of
itself; the other is the cause of all motion, and is either termed the
power of gravity, or of specific attraction, or the spirit of animation.
This immaterial agent is supposed to exist in or with matter, but to be
quite distinct from it, and to be equally capable of existence, after the
matter, which now possesses it, is decomposed.