up the ore and rubbish together, because we cannot
immediately distinguish them from each other. But the sooner we can
separate them, the better. In the beginning of all experimental
sciences, a number of useless particulars are recorded, because they
are not known to be useless; when, from comparing these, a few general
principles are discovered, the memory is immediately relieved, the
judgment and inventive faculty have power and liberty to work, and
then a rapid progress and great discoveries are made. It is the
misfortune of those who first cultivate new sciences, that their
memory is overloaded; but if those who succeed to them, submit to the
same senseless drudgery, it is not their misfortune, but their fault.
Let us look over the history of those who have made discoveries and
inventions, we shall perceive, that it has been by rejecting useless
ideas that they have first cleared their way to truth. Dr. Priestley's
Histories of Vision and of Electricity, are as useful when we consider
them as histories of the human mind, as when we read them as histories
of science. Dr. Priestley has published a catalogue of books,[49] from
which he gathered his materials. The pains, he tells us, that it cost
him to compress and abridge the accounts which ingenious men have
given of their own experiments, teach us how much our progress in real
knowledge depends upon rejecting all that is superfluous. When
Simonides offered to teach Themistocles the art of memory,
Themistocles answered, "Rather teach me the art of forgetting; for I
find that I remember much that I had better forget, and forget"
(_consequently_) "some things which I wish to remember."

When any discovery or invention is completed, we are frequently
astonished at its obvious simplicity. The ideas necessary to the
discovery, are seldom so numerous as to fatigue our memory. Memory
seems to have been useful to inventors only as it presented a few
ideas in a certain happy connection, as it presented them faithfully
and distinctly to view in the proper moment. If we wish for examples
of _the conduct of_ the understanding, we need only look into Dr.
Franklin's works. He is so free from all affectation, he lays his mind
so fairly before us, that he is, perhaps, the best example we can
select. Those who are used to look at objects in a microscope, say,
that full as much depends upon the object's being well prepared for
inspection, as upon the attention of the observer, or th