48 
MECHANICAL AND IMMECHANICAL PARTS 
.concerns not at all our knowledge of the mechanical parts 
of the same frame. I contend, therefore, that there is 
mechanism in animals; that this mechanism is as proper¬ 
ly such, as it is in machines made by art; that this me¬ 
chanism is intelligible and certain; that it is not the less 
so, because it often begins or terminates with something 
which is not mechanical: that whenever it is intelligible 
and certain, it demonstrates intention and contrivance, as 
well in the works of nature as in those of art; and that it 
is the best demonstration which either can afford. 
But whilst I contend for these propositions, I do not 
exclude myself from asserting, that there may be, and that 
there are, other cases, in which, although we cannot ex¬ 
hibit mechanism, or prove indeed that mechanism is em¬ 
ployed, we want not sufficient evidence to conduct us to 
the same conclusion. 
There is what may be called the chemical part of our 
frame; of which, by reason of the imperfection of our 
chemistry, we can attain to no distinct knowledge; I 
mean, not to a knowledge, either in degree or kind, similar 
to that which we possess of the mechanical part of our 
frame. It does not, therefore, afford the same species of 
argument as that which mechanism affords; and yet it may 
afford an argument in a high degree satisfactory. The gas¬ 
tric juice , or the liquor which digests the food in the stom¬ 
achs of animals, is of this class. Of all menstrua, it is the 
most active, the most universal. In the human stomach, for 
instance, consider what a variety of strange substances, and 
how widely different from one another, it, in a few hours, re¬ 
duces to a uniform pulp, milk, or mucilage. It seizes upon 
everything, it dissolves the texture of almost everything 
that comes in its way. The flesh of perhaps all animals; 
the seeds and fruits of the greatest number of plants; the 
roots, and stalks, and leaves of many, hard and tough as 
they are, yield to its powerful pervasion. The change 
wrought by it is different from any chemical solution which 
we can produce, or with which we are acquainted, in this 
respect as well as many others, that, in our chemistry, par¬ 
ticular menstrua act only upon particular substances. Con¬ 
sider, moreover, that this fluid, stronger in its operation 
than a caustic alkali or mineral acid, than red precipitate, 
or aqua-fortis itself, is nevertheless as mild, and bland, and 
inoffensive to the touch or taste, as saliva or gum-water, 
which it much resembles. Consider, I say, these several 
properties of the digestive organ, and of the juice with 
which it is supplied, or rather with which it is made to sup- 
