citability, as
perhaps the least liable to exception, and in using this term, it is
necessary to mention that I mean only to express a fact, without the
smallest intention of pointing out the nature of that property which
distinguishes living from dead matter; and in this we have the
illustrious example of Newton, who called that property which causes
bodies in certain situations to approach each other, gravitation,
without in the least hinting at its nature. Yet though he knew not
what gravitation was, he investigated the laws by which bodies were
acted on by it, and thus solved a number of phenomena which were
before inexplicable: in the same manner, though we are ignorant of
the nature of excitability, or of the property which distinguishes
living from dead matter, we can investigate the laws by which dead
matter acts upon living bodies through this medium. We know not what
magnetic attraction is, yet we can investigate its laws: the same may
be observed with respect to electricity. If ever we should obtain a
knowledge of the nature of this property, it would make no alteration
in the laws which we had before discovered.

Before we proceed to the investigation of the laws by which the
living principle or excitability is acted on, it will be first
necessary to define some terms, which I shall have occasion to use,
to avoid circumlocution: and here it may not be improper to observe,
that most of our errors in reasoning have arisen from want of strict
attention to this circumstance, the accurate definition of those
terms which we use in our reasoning. We may use what terms we please,
provided we accurately define them, and adhere strictly to the
definition. On this depends the excellence and certainty of the
mathematical sciences. The terms are few, and accurately defined; and
in their different chains of reasoning mathematicians adhere with the
most scrupulous strictness to the original definition of the terms.
If the same method were made use of in reasoning on other subjects,
they would approach to the mathematics in simplicity and in truth,
and the science of medicine in particular would be stripped of the
heaps of learned rubbish which now encumber it, and would appear in
true and native simplicity. Such is the method I propose to follow: I
am certain of the rectitude of the plan; of the success of the
reasoning it does not become me to judge.

When the excitability is in such a state as to be very susceptible of