To state a law in the form of a hypothesis serves to emphasize the fact that laws--what we have called natural laws at any rate--are subject to verification and restatement. Under the circumstances the exceptional instance, which compels a restatement of the hypothesis, is more important for the purposes of science than other instances which merely confirm it.
Any science which operates with hypotheses and seeks to state facts in such a way that they can be compared and verified by further observation and experiment is, so far as method is concerned, a natural science.
One thing that makes the conception of natural history and natural law important to the student of sociology is that in the field of the social sciences the distinction between natural and moral law has from the first been confused. Comte and the social philosophers in France after the Revolution set out with the deliberate purpose of superseding legislative enactments by laws of human nature, laws which were to be positive and "scientific." As a matter of fact, sociology, in becoming positive, so far from effacing, has rather emphasized the distinctions that Comte sought to abolish. Natural law may be distinguished from all other forms of law by the fact that it aims at nothing more than a description of the behavior of certain types or classes of objects. A description of the way in which a class, i.e., men, plants, animals, or physical objects, may be expected under ordinary circumstances to behave, tells us what we may in a general way expect of any individual member of that class. If natural science seeks to predict, it is able to do so simply because it operates with concepts or class names instead, as is the case with history, with concrete facts and, to use a logical phrase, "existential propositions."
That the chief end of science is descriptive formulation has probably been clear to keen analytic minds since the time of Galileo, especially to the great discoverers in astronomy, mechanics, and dynamics. But as a definitely stated conception, corrective of misunderstandings, the view of science as essentially descriptive began to make itself felt about the beginning of the last quarter of the nineteenth century, and may be associated with the names of Kirchhoff and Mach. It was in 1876 that Kirchhoff defined the task of mechanics as that of "describing completely and in the simplest manner the motions which take place in nature." Widening this a little, we may say that the aim of science is to describe natural phenomena and occurrences as exactly as possible, as simply as possible, as completely as possible, as consistently as possible, and always in terms which are communicable and verifiable. This is a very different rôle from that of solving the riddles of the universe, and it is well expressed in what Newton said in regard to the law of gravitation: "So far I have accounted for the phenomena presented to us by the heavens and the sea by means of the force of gravity, but I have as yet assigned no cause to this gravity.... I have not been able to deduce from phenomena the _raison d'être_ of the properties of gravity and I have not set up hypotheses." (Newton, _Philosophiae naturalis principia Mathematica_, 1687.)
