experience, or even to old
and familiar habits of thought. And this difficulty is a
necessary result of the fundamental laws of the human
mind. When we have often seen and thought of two things
together, and have never, in any one instance, either
seen or thought of them separately, there is by the
primary law of association an increasing difficulty,
which in the end becomes insuperable, of conceiving the
two things apart. This is most of all conspicuous in
uneducated persons, who are, in general, utterly unable
to separate any two ideas which have once become firmly
associated in their minds, and, if persons of cultivated
intellect have any advantage on the point, it is only
because, having seen and heard and read more, and being
more accustomed to exercise their imagination, they
have experienced their sensations and thoughts in more
varied combinations, and have been prevented from
forming many of these inseparable associations. But this
advantage has necessarily its limits. The man of the
most practised intellect is not exempt from the
universal laws of our conceptive faculty. If daily habit
presents to him for a long period two facts in
combination, and if he is not led, during that period,
either by accident or intention, to think of them apart,
he will in time become incapable of doing so, even by
the strongest effort; and the supposition, that the two
facts can be separated in nature, will at last present
itself to his mind with all the characters of an
inconceivable phenomenon. There are remarkable instances
of this in the history of science; instances in which
the wisest men rejected as impossible, because
inconceivable, things which their posterity, by earlier
practice, and longer perseverance in the attempt, found
it quite easy to conceive, and which every body now
knows to be true. There was a time when men of the most
cultivated intellects, and the most emancipated from the
dominion of early prejudice, could not credit the
existence of antipodes; were unable to conceive, in
opposition to old association, the force of gravity
acting upwards instead of downwards. The Cartesians long
rejected the Newtonian doctrine of the gravitation of
all bodies towards one another, on the faith of a
general proposition, the reverse of which seemed to th