ith more or less accuracy from experience the
law to which each of those causes, when acting separately, conforms,--and
have summed up the aggregate effect more or less carefully.

With all the precautions which have been indicated there will still be
some danger of falling into partial views; but we shall at least have
taken the best securities against it. All that we can do more, is to
endeavour to be impartial critics of our own theories, and to free
ourselves, as far as we are able, from that reluctance from which few
inquirers are altogether exempt, to admit the reality or relevancy of
any facts which they have not previously either taken into, or left a
place open for in, their systems.

If indeed every phenomenon was generally the effect of no more than one
cause, a knowledge of the law of that cause would, unless there was a
logical error in our reasoning, enable us confidently to predict all the
circumstances of the phenomenon. We might then, if we had carefully
examined our premises and our reasoning, and found no flaw, venture to
disbelieve the testimony which might be brought to show that matters had
turned out differently from what we should have predicted. If the causes
of erroneous conclusions were always patent on the face of the
reasonings which lead to them, the human understanding would be a far
more trustworthy instrument than it is. But the narrowest examination of
the process itself will help us little towards discovering that we have
omitted part of the premises which we ought to have taken into our
reasoning. Effects are commonly determined by a _concurrence_ of causes.
If we have overlooked any one cause, we may reason justly from all the
others, and only be the further wrong. Our premises will be true, and
our reasoning correct, and yet the result of no value in the particular
case. There is, therefore, almost always room for a modest doubt as to
our practical conclusions. Against false premises and unsound reasoning,
a good mental discipline may effectually secure us; but against the
danger of _overlooking_ something, neither strength of understanding nor
intellectual cultivation can be more than a very imperfect protection.
A person may be warranted in feeling confident, that whatever he has
carefully contemplated with his mind's eye he has seen correctly; but no
one can be sure that there is not something in existence which he has
not seen at all. He can do no more than satisfy himself that