f a truth being
derived from experience, we imply the usual exercise of our mental
faculties; it is the step from a general to a universal proposition
which alone occasions this perplexing distinction. The dogma is
this--that experience can only teach us by a limited number of
examples, and therefore can never establish a universal proposition.
But if _all_ experience is in favour of a proposition--if no
experience has occurred even to enable the imagination to conceive its
opposite, what more can be required to convert the general into a
universal proposition?

Strange to say, the attribution of these characteristics of
universality and necessity, becomes, amongst those who loudly insist
upon the palpable nature of the distinction we are now examining, a
matter of controversy; and there are a class of scientific truths, of
which it is debated whether they are contingent or necessary. The
only test that they belong to the latter order is, the impossibility
of conceiving their opposites to be the truth; and it seems that men
find a great difference in their powers of conception, and that what
is impossible with one is possible with another. But (wisely, too)
passing this over, and admitting that there is a distinction (though
a very ill-defined one) between the several truths we entertain of
this nature; namely, that some we find it impossible, even in
imagination, to contradict, whilst of others we can suppose it
possible that they should cease to be truths--does it follow that
different faculties of the mind are engaged in the acquisition of
them? Does nothing depend on the nature of the subject itself? "That
two sides of a triangle," says Mr Whewell, "are greater than the
third, is a universal and necessary geometrical truth; it is true of
all triangles; it is true in such a way that the contrary cannot be
conceived. _Experience could not prove such a proposition._"
Experience is allowed to prove it of this or that triangle, but not
as an inseparable property of a triangle. We are at a loss to
perceive why the same faculties of the mind that can judge, say of
the properties of animal life, of organized beings, cannot judge of
the properties of a figure--properties which must immediately be
conceived to exist the moment the figure is presented to the
imagination. We say, for instance, of any animal, not because it is
this or that animal, a sheep or an ox, but simply _as_ animal, that
it must sustain itself by food, b