Francis Bacon drew attention to these inner perverting influences, these universal sources of erroneous belief, in his _De Augmentis_ and again in his _Novum Organum_, under the designation of _Idola_ ([Greek: eidola]), deceptive appearances of truth, illusions. His classification of _Idola--Idola Tribus_, illusions common to all men, illusions of the race; _Idola Specus_, personal illusions, illusions peculiar to the "den" in which each man lives; _Idola Fori_, illusions of conversation, vulgar prejudices embodied in words; _Idola Theatri_, illusions of illustrious doctrine, illusions imposed by the dazzling authority of great names--is defective as a classification inasmuch as the first class includes all the others, but like all his writings it is full of sagacious remarks and happy examples. Not for the sake of novelty, but because it is well that matters so important should be presented from more than one point of view, I shall follow a division adapted from the more scientific, if less picturesque, arrangement of Professor Bain, in his chapter on the Fallacious Tendencies of the Human Mind.[1] The illusions to which we are all subject may best be classified according to their origin in the depths of our nature. Let us try to realise how illusory beliefs arise. What is a belief? One of the uses of Logic is to set us thinking about such simple terms. An exhaustive analysis and definition of belief is one of the most difficult of psychological problems. We cannot enter upon that: let us be content with a few simple characters of belief. First, then, belief is a state of mind. Second, this state of mind is outward-pointing: it has a reference beyond itself, a reference to the order of things outside us. In believing, we hold that the world as it is, has been, or will be, corresponds to our conceptions of it. Third, belief is the guide of action: it is in accordance with what we believe that we direct our activities. If we want to know what a man really believes, we look at his action. This at least is the clue to what he believes at the moment. "I cannot," a great orator once said, "read the minds of men." This was received with ironical cheers. "No," he retorted, "but I can construe their acts." Promoters of companies are expected to invest their own money as a guarantee of good faith. If a man says he believes the world is coming to an end in a year, and takes a lease of a house for fifteen years, we conclude that his belief is not of the highest degree of strength.