We come now to consider two conditions to which every natural science must be subject, and which may therefore serve as tests of its fitness to be regarded as a “Science.” The first of these is a capability of infinite progress in each of its elements without detriment to its integrity as a whole.
We have already seen that the science of Physics is capable of such progress in the phenomena with which it deals. Our knowledge of these phenomena is continually growing more comprehensive and more minute, and new elements of knowledge are continually coming into our possession with regard to them; yet their relation to each other through the law of attraction remains the same, and the science, in its nature and structure, receives no modification. But not only are its subjects, the phenomena, capable of progress; the law itself may advance from a minor to a grander generalization, as it was advanced, step by step, by Newton, and still without destruction of the previous steps. The same is true of Chemistry. As our means of investigation become more exact and extensive, our knowledge of the properties of bodies increases; yet the law of Chemical Affinity in definite proportions remains undisturbed. Furthermore, this law itself becomes the subject of more extended generalizations. The law as established by Black and Cavendish was suspended, yet not overthrown (nor the science disturbed) by the wider generalization of Dalton’s Atomic Theory; and this again forms but a part of Faraday’s Theory of the Identity of Chemical Affinity and Electrical Attraction, and amid all these revolutions in abstract theory, the science of practical Chemistry has held an even way.
In the science of Optics, again, the phenomena of the luminous body and those of light-receiving body are connected by the law of the transmission and diffusion of light. This structure of the science and all that is based upon it rests undisturbed through all the mighty changes which recent undisturbed through all the mighty changes which recent discoveries have wrought in our knowledge of Optics-enriched but not revolutionized by them. But while recent discoveries have wrought in our knowledge of Optics-enriches but not revolutionized by them. But while the progress of the science has been thus rapid and uninterrupted, philosophers have been and remain at variance respecting the very nature of light and its mode of propagation. One school holds to the theory of emission, another to the undulatory theory.
Now, let us suppose that the science of Optics, instead of being based upon an empirical law, the result of induction, and which expressed the relation between two series of observation phenomena, had been based upon a theory of the essential nature of light and its mode transmission. It is very evident that the whole structure that might be built upon the theory of emission would be toppled over so soon as the theory of undulation should be established. And should subsequent observation again lend countenance to the theory of emission, down would go, of necessity, the whole science as built upon the undulatory theory. And thus, inasmuch as experience and sound philosophy render it tolerably certain that we shall never attain absolute knowledge on this subject, we should have, through all time, an endless succession of half-completed fabrics and hopeless ruins, but no infinitely progressive science of Optics.
This furnishes an exact illustration of what would be the history of any science of Therapeutics not constructed upon the empirical plan already elaborated. Let us suppose that instead of taking as the subject of action, the phenomena of disease as derived from observation with all the aid which auxiliary sciences can afford, we should take a theory of disease, its essential nature and mode of development, and upon this theory should show that our theory was insufficient and untenable (and facts would show this, since, as we have already seen, the essential nature of disease is inscrutable). Now our whole structure would be overthrown, nor would the fragments even be available for the construction of another science or another theory, for they would consist of observations made, not independently, but on the basis of a theory and with reference to it. There could thus be no possibility of steady, uninterrupted, infinite progress in the science. There would be a succession of dynasties of theory, each based on the overthrow of a former, and each in turn demolished by its successor. That this is the history of the Therapeutics of the Old School of medicine up to the present day is admitted by common consent. It will continue until attempts to erect of rational science of Therapeutics upon a theory of disease, confounding the law with the phenomena, shall give place to the construction of an empirical inductive science.
“What the!” it may be asked, “shall we not investigate and theorize upon the nature of disease in general, or of diseases in particular?” By all means investigate and theorize, just as physicists do upon the nature, properties and transmission of light-but do it as studying the science of Pathology, not that of Therapeutics-and avoid most carefully, making these theories which can be at best no more than temporary aids to the observation and grouping of phenomena, the basis of a practical science. If, however, they throw light on the study of phenomena, render out observations of them more keen and more exact, or afford us connecting links between isolated groups of phenomena, and they will do all this, they will render valuable indirect aid to the science of Therapeutics, just as similar investigations of light have advanced our knowledge of Optics.
A second conditions or test of a natural science, and therefore of any proposed science of Therapeutics, is that it shall provide for the prediction of future events within its own domain. It must furnish means of prevision. The problem must be as follows: Given the law and one series of phenomena, to state the corresponding series of phenomena on the other side. This condition is admirably stated by Whewell: “Men cannot help believing that the laws laid down by discoverers must be in a great measure identical with the real laws of nature, when the discoverers thus determine effects beforehand, in the same manner in which nature herself determines them when the occasion occurs. Those who can do this must to a great extent have detected nature’s secret-must have fixed upon the conditions to which she attends and must have seized the rules by which she applies them. Such a coincidence of untried facts with speculative assertions cannot be the work of chance, but implies some large portion of truth in the principles on which the reasoning is founded.
To trace order and law in that which has been observed, may be considered as interpreting what nature has written down for us and will commonly prove that we understand her alphabet. But to predict what has not been observed is to attempt ourselves to use the legislative phrases of nature; and when she responds plainly and precisely to that which we thus utter, we cannot but suppose that we have in a great measure made ourselves masters of the meaning and structure of her language. The prediction of results even of the same kind as those which have been observed, in new cases, is a proof of real success in our inductive processes.” (1 Whewell, Philosophy of the Inductive Sciences, vol.ii., pp.64,65).
The discovery of the planet Neptune by Leverrier and Adams affords an instance of this prevision, in Astronomy, the details of which are familiar to all. The problem was: Given the law of attraction and a series of phenomena, consisting of certain unexplained perturbations of Jupiter-to find the other and corresponding series of phenomena. The calculations were made; the resulting phenomena were stated to be those of an undiscovered planet of a given size and orbit and a definite location, the existence of which was accordingly affirmed and its discovery predicted. Physical investigation confirmed the induction.
This condition applies to all natural sciences, and of course to Therapeutics. The problem would be: Given the Therapeutics law and a certain series of phenomena of natural or drug disease, to find the corresponding series of phenomena of drug or natural disease.