90. Stages of the Inductive Process. -- To perform an induction is to ascend from effects to their cause, to determine the properties, and, through them, the nature of the cause, in order to understand the law of its action. Induction comprises four stages:

(1) The observation of certain facts which are presented to the senses. E. g., the chemist observes, a certain number of times, in dissimilar circumstances, that different absolute quantities of hydrogen (H) and of chlorin (Cl) have combined to form a definite body (HCl).

(2) The hypothesis -- The investigator supposes that the observed phenomenon is inexplicable by constantly recurring fortuitous coincidences, and that it must have a sufficient reason in the nature of reacting bodies.

A scientific hypothesis is the provisional explanation of certain observed facts.^{1}

(3) The verification of the hypothesis, which is the heart of the inductive process, is effected sometimes by simple observation, sometimes, in a decisive manner, by experiment.

(a) By observation. The author of an hypothesis imagines the results that should follow if it were verified and found to operate in nature.

(b) By experiment. The observer is not merely a witness of the course of events in nature, but himself influences those events. By artificial means he varies, according to the aim which he has in view, the agents which operate in a complex phenomenon.

(4) The deduction. -- The property of the bodies Cl and H, to combine in the definite proportions of 1 and 35.5, once recognized, the reasoning faculty goes further and from its verified observations draws a general conclusion: thenceforward, every time H and Cl are mixed in the proportions of 1 and 35.5, and exposed to the action of the sun's rays, hydrochloric acid will be formed, and 22 calories per molecule-gramme will be liberated.

The consideration of these several stages raises various questions: (1) the observation of facts and their verification by experiment belongs to the description of the methods of induction; (2) the generalization of the observed fact raises the question of the basis of induction; (3) as to deduction, or the last stage of induction, we shall note the relations between deduction and induction.

{1} See the thorough study of hypothesis in the Higher Course.