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 THEORY OF KNOWLEDGE. 167 edge of things ! If this is possible it can only be indirect knowledge — the mind knows things through its ideas, and possesses criteria which show that its ideas agree with things. Two cases must be clearly distinguished, for a consider- able number of our ideas, viz., all complex ideas except those of substances, make no claim to represent things, and consequently cannot represent them falsely. For mathe- matical and moral ideas and principles, and the truth thereof, it is entirely immaterial whether things and condi- tions correspondent to them exist in nature or not. They are valid, even if nowhere actualized ; they are " eternal truths," not in the sense that they are known from child- hood, but in the sense that, as soon as known, they are immediately assented to.* The case is different, however, with simple ideas and the ideas of substances, which have their originals without the mind and which are to corre- spond with these. In regard to the former we may always be certain that they agree with real things, for since the mind can neither voluntarily originate them {e. g., cannot pro- duc e^sensation s 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 produc- tions 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 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.
 * Thus it results that knowledge, although dependent on experience for all