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 theory only after such a verification has been made; and it may be considered as a natural law only after it has stood the above test, and the cause, the reason of the hypothesis, has been discovered. Thus we describe Newton's induction of universal gravitation as a proved theory, because it has been confirmed by an enormous mass of facts, many of which seemed to be contradictory at first sight. And although we often speak also of the law of gravitation, this way of speaking is not quite correct, as the cause of this universal fact of gravitation is not yet discovered : it is only foreseen.

Jacobins, or Jacobinists, name given to the members of a club, consisting of middle-class Radicals, which played a prominent part in the Great French Revolution of 1789–1794. It contained in its midst most of the prominent Republican revolutionists, and was very much under the influence of Robespierre. It courageously fought against the royal power; but after that fell, it also struggled against the Club of the Cordeliers, to which belonged Danton, Hébert, and the most influential members of the Commune of Paris. During the period of the Terror, the Jacobin Club became a sort of Grand Jury of accusation. After the fall of Robespierre and his party, in July, 1794, it was closed. The word "Jacobinist" is now used to describe the advocates of a powerful, centralised revolutionary Government.

Joule, James Prescott (1818–1889). English physicist; determined the mechanical equivalent of heat. (See note on that subject.)

Kant, Immanuel (1724–1804), German philosopher, whose philosophy exercised the deepest influence in the nineteenth century. In his earlier works he devoted himself chiefly to natural science, and almost at the same time as Laplace he formulated a hypothesis, quite similar to that of Laplace, of the origin of our solar system from a cooling mass of incandescent gases. His principal work, however, was his "Critique of Pure Reason." There are, according to him, two different worlds: (1) the world of physical phenomena, which we can know in space and time, but which, in accordance with his philosophy of critical, transcendental idealism, are mere phenomena having no reality in themselves; and (2) the world of inborn ideas, "the things in themselves" (Dinge an sich), which we can know in time, but not in space. The enigma of a world of "things in themselves" existing behind the phenomena, he tried to solve through the moral philosophy ("Critique of Practical Reason"). In this second great work he endeavoured to prove that Reason possesses the property of dictating to itself its own laws, and that it is the duty of the moral man to follow these prescriptions of his reason—"the categorical imperative." Upon the idea of the moral conscience he based the ideas of God, Immortality, and Liberty. In his philosophy of Law he developed the idea that an absolute respect for moral liberty had to be the foundation of all society and State-life, and in the realisation of this ideal of liberty he saw the future aim of all historical development.