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distinct ideas are separable in thought, assumes the question at issue.

But the question whether the idea of causation is necessary, or not, is
really of very little importance. For, to say that an idea is necessary
is simply to affirm that we cannot conceive the contrary; and the fact
that we cannot conceive the contrary of any belief may be a presumption,
but is certainly no proof, of its truth.

In the well-known experiment of touching a single round object, such as
a marble, with crossed fingers, it is utterly impossible to conceive
that we have not two round objects under them; and, though light is
undoubtedly a mere sensation arising in the brain, it is utterly
impossible to conceive that it is not outside the retina. In the same
way, he who touches anything with a rod, not only is irresistibly led to
believe that the sensation of contact is at the end of the rod, but is
utterly incapable of conceiving that this sensation is really in his
head. Yet that which is inconceivable is manifestly true in all these
cases. The beliefs and the unbeliefs are alike necessary, and alike
erroneous.

It is commonly urged that the axiom of causation cannot be derived from
experience, because experience only proves that many things have causes,
whereas the axiom declares that all things have causes. The syllogism,
"many things which come into existence have causes, A has come into
existence: therefore A had a cause," is obviously fallacious, if A is
not previously shown to be one of the "many things." And this objection
is perfectly sound so far as it goes. The axiom of causation cannot
possibly be deduced from any general proposition which simply embodies
experience. But it does not follow that the belief, or expectation,
expressed by the axiom, is not a product of experience, generated
antecedently to, and altogether independently of, the logically
unjustifiable language in which we express it.

In fact, the axiom of causation resembles all other beliefs of
expectation in being the verbal symbol of a purely automatic act of the
mind, which is altogether extra-logical, and would be illogical, if it
were not constantly verified by experience. Experience, as we have seen,
stores up memories; memories generate expectations or beliefs--why they
do so may be explained hereafter by proper investigation of cerebral
physiology. But, to seek for the reason of the facts in the verbal
symbols by which they are expressed, and to be a