ferent in different
living systems; but are in general the following; sense and motion in
almost all animals, and in many the power of thinking, and other
affections of the mind.

The powers, or dead matters, which by their action produce these
functions, are chiefly heat, food, and air. The proof that these
powers do produce the living functions is in my opinion very
satisfactory, for when their action is suspended, the living
functions cease. If we take away, for instance, heat, air, and food,
from animals, they soon become dead matter. This is as strong a proof
that these matters are the cause of the functions, as that heat is
the cause of the expansion of bodies, when we find that by
withdrawing it the expansion ceases. Indeed it is not necessary that
an animal should be deprived of all these powers to put a stop to the
living functions; if any one of them is taken away, the body sooner
or later becomes dead matter: it is found by experience, that if a
man is deprived of air, he dies in about three or four minutes; for
instance, if he is immersed under water: if he is deprived of heat,
or in other words is exposed to a very severe degree of cold, he
likewise soon dies; or if he is deprived of food, his death is
equally certain, though more slow; it is sufficiently evident then
that the living functions are owing to the action of these external
powers upon the body.

What I have here said is not confined to animals, but the living
functions of vegetables are likewise caused by the action of dead
matter upon them. The powers, which by their actions produce the
living functions of vegetables, are principally heat, moisture,
light, and air.

From what has been said, it clearly follows, that living bodies must
have some property different from dead matter, which renders them
capable of being acted on by these external powers, so as to produce
the living functions; for if they had not, it is evident that the
only effects which these powers could produce, would be mechanical,
or chemical.

Though we know not exactly in what this property consists, or in what
manner it is acted on, yet we see that when bodies are possessed of
it, they become capable of being acted on by external powers, so as
to produce the living functions.

We may call this property, with Haller, irritability, or, with Brown,
excitability; or we may use vital principle, or any other term, could
we find one more appropriate. I shall use the term ex