for it. In like manner, we have sufficient reason, judging merely from the observation of previous speculation, to warrant us in abstaining from attempts to deduce a knowledge of material nature from mental concepts. Add to this fact that other, that all knowledge has its basis in experience, and we supplement our empirical conclusion by a deduction from a fundamental law of nature, and show their entire congruence.
Early physical speculation, with hardly an exception, proceeds on the assumption that knowledge is derived primarily from the mind—thus completely inverting the true order. Would some of those persons who in our day attribute so much power to the unaided mind carefully peruse a history of science in the early centuries of our era and the centuries immediately preceding it, we think they would become convinced that purely mental speculation can never lead to any exact knowledge, but must, on the contrary, invariably be a source of obscurity and error.
With but two or three exceptions, the most eminent men of the early and middle ages, except as in a few instances they recorded interesting facts, contributed absolutely nothing to scientific progress. The principal cause of the difference in the civilization of our times and those of the ancients is the progress of exact knowledge. What would the most mystical of metaphysicians of to-day say to Socrates's assertion that those things are called like which partake of the quality of likeness? or to Aristotle's argument against a void in nature, that a void is a negation, and in a negation there could be no differences, and where there were no differences there could be no up or down, consequently bodies could not move up or down in a void, but it is the nature of bodies to move up and down, therefore there is no void? or to his argument in support of circular motion, that it is the best, therefore the most natural? The mental ability of the "Father of Logic" is unquestioned. He follows logical methods closely enough. What, then, is there unscientific in his reasoning? Simply that he allows abstract terms and mental notions to pass current for facts—thus using an irredeemable currency as though it possessed intrinsic value.
The modern astronomer can not but wonder at the daring speculations of the Pythagoreans, that, as ten was a perfect number, there must be ten heavenly bodies, notwithstanding nine only were then known. Before such vaticinations the predictions of a Leverrier sink into insignificance.
We find the whole history of science, up to the time of Copernicus, a history of the conflict of facts with preëstablished notions. The problem of astronomers was to reconcile the apparent motions of the heavenly bodies with their assumed circular motions. Thus, when it was found that the motions of the planets were not uniformly circular, it was naïvely suggested that an uncertain motion could not be
