ce the mind can neither
voluntarily originate them (_e.g._, cannot produce sensations of color
in the dark) nor avoid having them at will, but only receive them from
without, they are not creatures of the fancy, but the natural and regular
productions of external things affecting us. In regard to the latter, the
ideas of substances, we may be certain at least when the simple ideas which
compose them have been found so connected in experience. Perception has
an external cause, whose influence the mind is not able to withstand. The
mutual corroboration furnished by the reports of the different senses, the
painfulness of certain sensations, the clear distinction between ideas from
actual perception and those from memory, the possibility of producing and
predicting new sensations of an entirely definite nature in ourselves and
in others, by means of changes which we effect in the external world (e.g.
by writing down a word)--these give further justification for the trust
which we put in the senses. No one will be so skeptical as to doubt in
earnest the existence of the things which he sees and touches, and to
declare his whole life to be a deceptive dream. The certitude which
perception affords concerning the existence of external objects is indeed
not an absolute one, but it is sufficient for the needs of life and the
government of our actions; it is "as certain as our happiness or misery,
beyond which we have no concernment, either of knowing or being." In regard
to the past the testimony of the senses is supplemented by memory, in
which certainty [in regard to the continued existence of things previously
perceived] is transformed into high probability; while in regard to the
existence of other finite spirits, numberless kinds of which may be
conjectured to exist, though their existence is quite beyond our powers of
perception, certitude sinks into mere (though well-grounded) faith.

[Footnote 1: Thus it results that knowledge, although dependent on
experience for all its materials, extends beyond experience. The
understanding is completely bound in the reception of simple ideas; less so
in the combination of these into complex ideas; absolutely free in the act
of comparison, which it can omit at will; finally, again, completely bound
in its recognition of the relation in which the ideas it has chosen
to compare stand to one another. There is room for choice only in the
intermediate stage of the cognitive process; at the be