Reference has already been made to the importance of the process of comparison in the mental analysis of observed phenomena. The observation of simi larities and differences, aided by the processes of analysis and syn thesis, is one of the first steps to knowledge of every kind, and continues to be indispensable to the pursuit of science throughout its progress. But there are degrees of similarity. Things may be so alike that they are at once treated as instances of the same kind or class. And the formulation and application of generaliza tions of all kinds are based upon this possibility of apprehending such class resemblances. On the other hand, there is a likeness which stops short of such close class likeness. Such similarity is usually called analogy. The term is applied to similarity of struc ture or of function or of relationship, in fact, to similarity of almost every kind except that which characterizes members of the same class, in the strict sense of the term. And analogy plays a very important part in the work of science, especially in suggest ing those suppositions or hypotheses which, as already explained, are so essential to scientific research and discovery. As the subject is discussed in the article ANALOGY, no more need be said about it here.
After this brief survey of various mental activities which are more or less involved in the pursuit of every kind of knowledge, and consequently form no suitable bases for the differentiation of the various methods of science, we may now proceed to the con sideration of the several scientific methods properly so called.
This may be described as the oldest and simplest of scientific methods. The observation of similarities be tween certain things, and classing them together, marks the earliest attempt to discover some kind of order in the apparently chaotic jumble of things that confront the human mind. Language bears witness to the vast number of classifications made spontaneously by pre-scientific man. For every common noun expresses the recognition of a class and language is much older than science. The first classifications subserved strictly practical purposes, and had reference mainly to the uses which man could make of the things classified. They were frequently also based on superficial resemblances which veiled deeper differences, or were influenced by superficial differences which diverted attention from deeper similarities. But with the growth of the scientific spirit classifica tions became more objective or more natural, attention being paid to the objective nature of the things themselves rather than to their human uses. Even now scientific classification rarely begins at
the beginning, but sets out from current classifications embodied in language. It has frequent occasion to correct popular classifica tions. At the same time it has difficulties of its own, and more than one science has been held up for centuries for want of a really satisfactory scheme ox classification of the phenomena constituting its field of investigation. To recognize a class is to recognize the unity of essential attributes in a multiplicity of instances; it is a recognition of the one in the many. To that extent it is a dis covery of order in things. And although it is the simplest method of science, and can be applied before any other method, it is also the fundamental method, inasmuch as its results are usually as sumed when the other methods are applied. For science is not, as a rule, concerned with individuals as such, but with kinds or classes. This means that the investigator usually assumes the accuracy of the classification of the phenomena which he is study ing. Of course, this does not always turn out to be the case. And the final outcome of the application of other methods of science to certain kinds of phenomena may be a new classification of them.
When classifying things according to their present similarities and differences it sometimes happens that one is impressed by certain resemblances between things, although these cannot be regarded as belonging to the same class. In such cases the suggestion sometimes forces itself upon the mind of the investigator that these similar yet different classes of things may be descendants of a common an cestry. And, if he inclines to the general feasibility of the idea of evolution in some sense or other, he will feel confronted with a new problem, namely, that of tracing the stages in the descent or evolution of the kindred classes. This applies especially to the study of living things and their creations, including human cus toms, institutions, discoveries and inventions. This method of investigation is commonly known as the genetic or evolutionary method. Wherever it is applied with success it affords a deeper insight into the unity and continuity of certain kinds of facts than is afforded by the above-mentioned method of classification. Sometimes, indeed, the real significance of the resemblances on which ordinary classification is based is brought to light first by the genetic method, which shows the similar classes to be different stages or phases of a system of continuous or intimately connected differentiations.