HYPOTHESIS. In endeavoring to explain natural phenomena, we have often to assume or imagine a cause, which, in the first instance, we do not know to be the real cause, but which may be established as such when we find that its consequences agree with the phenomenon to he explained. Every genuine theory was at one stage a mere conjecture, and became a true theory in consequence of being proved or verified by the proper methods. Thus, when it occurred to Newton that the force of gravity on the earth, as exemplified in falling bodies, might extend to the distance of the moon, and might be the power that compelled it to circle round the earth, instead of going off in a straight line through space, the suggestion was only an hypothesis, until such time as he was able to show that it accounted exactly for the facts, and then it became a theory.
A difference of opinion has arisen as to what constitutes a legitimate hypothesis, there being manifestly some necessary limits to the process of imagining possible causes. Time case that has chiefly contributed to make this a question is the celebrated undula tory theory of light, a theory, or hypothesis rather, remarkable not only for the extent to which it explains the facts, but for having led to the discovery of new facts by way of inference from the theory itself. Notwithstanding all this amount of coincidence, the ethereal substance whose undulations are supposed to constitute light in its passage from the sun to the earth, is not known to have a real existence. It is an imaginary element, so happily conceived as to express with fidelity a series of extremely compli cated phenomena. This was not the character of Newton's hypothesis as to the motion of the moon; the power supposed by him (the earth's gravity) was an actual or existing force, and all he did was to suggest that it extended as far as the moon. Accordingly, M. Auguste Comte and Mr. J. S. Mill have laid it down as the condition of a sound scientific hypothesis that the cause assigned to the phenomenon in question should be either a real cause, or capable of being ascertained to be a real cause, and that the liberty given to the scientific inquirer should be confined to imagining its operation in a particu lar sphere, and the law and amount of its operation, since both these could be verified by experiment and calculation. On the other hand, Dr. Whewell has contended that an amount of agreement with observed facts, such as has been exemplified by the undula tory hypothesis, is sufficient to establish not merely MI hypothesis, but a theory, at least until such a time as some discordant facts arise, when the theory must he modified or abandoned. But whatever name be given to this class of suppositions, it is evident that
they must he deemed inferior in scientific value to time other class of suppositions,where no cause or agent is assumed but what is actually known to exist, and where the only question is the presence of that agent in such manner and amount as to tally with the observed facts. Gravity, heat, electricity, moo established natural agents, and when we assume any one of these as the cause of some phenomena, we are on safe ground so far, that if it be once shown that they are actually operative in the case we arc dealing with, and that their calculated effect exactly coincides with the observed effect, the explanation is complete and final; no subsequent discovery can disturb a conclusion established in this way. But if we have to assume the very agency itself, or to imagine a power that we have no experience of, the coincidence between the laws of the assumed agency and the laws of the phenomena produces at best but a temporary or provisional evidence, which is liable to be superseded whenever a still better imagined machinery shall be brought forward. Thus, in the case of light, the first hypothesis, that of Newton himself, was it stream or shower of corpuscles; this gave way to the undulatory ether, whose merit lay in embracing the facts more closely; but we have no security against the ultimate preference of sonic third supposition which shall displace the second, as that did the first; while, perhaps, a day may come when an agency shall be proved to exist capable of explaining the phenomena. may granting that we must sometimes assume an unknown agent (when an effect seems to be beyond the power of all the recognized forces), yet, in ordinary researches, it is considered a grave objection if the assumed agent he of such a subtle or occult nature, or so far removed from observation, that its existence does not admit of being proved. Such was the doctrine of the Cartesian vortices, and such are any hypotheses as to the shapes, sizes, and dis tances of the ultimate atoms of matter. Such also is the doctrine of nervous fluids, whereby the impulses of mind are supposed to be propagated between the brain and the other parts of the body.