Certitude is the knowledge of a truth:—but what is once true is always true, and cannot fail, whereas what is once known need not always be known, and is capable of failing. It follows, that if I am certain of a thing, I believe it will remain what I now hold it to be, even though my mind should have the bad fortune to let it drop. Since mere argument is not the measure of assent, no one can be called certain of a proposition, whose mind does not spontaneously and promptly reject, on their first suggestion, as idle, as impertinent, as sophistical, any objections which are directed against its truth. No man is certain of a truth, who can endure the thought of the fact of its contradictory existing or occurring; and that not from any set purpose or effort to reject that thought, but, as I have said, by the spontaneous action of the intellect. What is contradictory to the truth, with its apparatus of argument, fades out of the mind as fast as it enters it; and though it be brought back to the mind ever so often by the pertinacity of an opponent, or by a voluntary or involuntary act of imagination, still that contradictory proposition and its arguments are mere phantoms and dreams, in the light of our certitude, and their very entering into the mind is the first step of their going out of it. Such is the position of our minds towards the heathen fancy that Enceladus lies under Etna; or, not to take so extreme a case, that Joanna Southcote was a messenger from heaven, or the Emperor Napoleon really had a star. Equal to this peremptory assertion of negative propositions is the revolt of the mind from suppositions incompatible with positive statements of which we are certain, whether abstract truths or facts; as that a straight line is the longest possible distance between its two extreme points, that Great Britain is in shape an exact square or circle, that I shall escape dying, or that my intimate friend is false to me.