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 beginning (in the reception of the simple ideas of perception, a, b, c, d), and at the end (in judging how the concepts a b c and a b d stand related to each other), the understanding is completely determined.]