to nature is made but once and at the start, in physics new appeals to experience must often be made in the course of the reasoning, and the final relations are accepted only when they are not contradicted by the order of nature. And sometimes, even if the facts do not support the theory, it is still important to observe that the conditions of experiment may not have been so simple as the premises assumed and that the theory may still be true when the proper limitations are introduced. In the minds of certain men who are pleased to call themselves practical, a theory is exploded and to be completely rejected as soon as any discrepancy appears between the observed facts and the reasoned conclusions. To the physicist, however, some sort of theory or rational guide is so important and even necessary that a very imperfect or insufficient theory is preferable to none at all. He is not one of those who delight to hold up to ridicule false and abandoned theories, of which so many examples may be found in the history of physics, for he truly recognizes that though they now seem absurdly wrong, they nevertheless served a useful purpose as a temporary scaffold, without whose aid the more lasting structure might never have been erected. From familiar acquaintance with the imperfections of all experimental data, the physicist grows into the habit of holding his deductions subject to correction in the light of new or more accurate observations. Thus there arises the idea so well expressed by the late Professor Rowland, of degrees of truth or untruth.
The ordinary crude mind has only two compartments, one for truth and one for error; indeed the contents of the two compartments are sadly mixed in most cases; the ideal scientific mind has an infinite number. Each theory or law is in its proper compartment indicating the probability of its truth. As a new fact arrives the scientist changes it from one compartment to another so as if possible to always keep it in its proper relation to truth and error.
The aim of physical science, according to the earlier writers, was the explanation of the phenomena of nature, i. e., the tracing of occurrences to their causes or the proceeding by logical advance from the cause to the effect. The modern and more acceptable view, due perhaps in large measure to Kirchhoff, is that science aims to state in simple and easily reproducible language the order of the processes of nature. A phenomenon then has received its full explanation when we have presented to the mind a picture or a model in which we may reproduce at will the sequence of events which is observed in nature. All attempts beyond this to satisfy the sense of causation must be futile.
It is a well-known fact that the mind derives a certain pleasure in