(1) In the sciences of reasoning, to discover ingenious methods of demonstration, stratagems for avoiding or overcoming difficulties.
(2) In the experimental sciences for inventing methods of research or of control--whence its analogy, above mentioned, to the practical imagination. Furthermore, the reciprocal influence of these two forms of imagination is a matter of common observation: a scientific discovery permits the invention of new instruments; the invention of new instruments makes possible experiments that are increasingly more complicated and delicate.
One remark further: This constructive imagination at the third stage is the only one met with in many scientists. They lack genius for invention, but discover details, additions, corrections, improvements. A recent author distinguishes (a) those who have created the hypothesis, prepared the experiments, and imagined the appropriate apparatus; (b) those who have imagined the hypothesis and the experiment, but use means already invented; and (c) those who, having found the hypothesis made and demonstrated, have thought out a new method of verification.[116] The scientific imagination becomes poorer as we follow it down this scale, which, however, bears no relation to exactness of reasoning and firmness of method.
Neglecting species and varieties, we may reduce the fundamental characters of the scientific imagination to the following:
For its material, it has concepts, the degree of abstraction of which varies with the nature of the science.
It employs only those associational forms that have an objective basis, although its mission is to form new combinations, "the discoveries consisting of the relation of ideas, capable of being united, which hitherto have been isolated."[117] (Laplace.) All association with an affective basis is strictly excluded.
It aims toward objectivity: in its conjectural construction it attempts to reproduce the order and connection of things. Whence its natural affinity for realistic art, which is midway between fiction and reality.
It is unifying, and so just the opposite of the esthetic imagination, which is rather developmental. It puts forward the master idea (Claude Bernard's _idée directrice_), a center of attraction and impulse that enlivens the entire work. The principle of unity, without which no creation succeeds, is nowhere more visible than in the scientific imagination. Even when illusory, it is useful. Pasteur, scrupulous scientist that he was, did not hesitate to say: "The experimenter's illusions are a part of his power: they are the preconceived ideas serving as guides for him."