"Qualities called sensible" [said Hobbes] "are, in the object that causeth them, but so many several motions of the matter by which it presseth our organs diversely.... Because the image in vision, consisting of color and shape, is the knowledge we have of the qualities of the object of that sense, it is no hard matter for a man to fall into this opinion, that the same color and shape are the very qualities themselves." But, concluded Hobbes: "The subject wherein color and image are inherent is not the object or thing seen.... There is nothing without us (really) which we call an image or color.... The said image or color is but an apparition unto us of _the motion, agitation, or alteration, which the object worketh in the brain, or spirits, or some internal substance of the head_.... As in vision, so also in conceptions that arise from the other senses, the subject of their inference is not the object, but the sentient." When John Locke began his great "Essay" on _The Human Understanding_, and posited mind in its first estate as a passive nonentity--a "blank tablet"--he had no vital conception of scientific idealism. But, in the patient thinking of twenty years, such a man could not fail to come upon the law, though he saw it only in part, and did not work it out. This work was carried a great way beyond him, by the acute and learned Bishop Berkeley, who showed from practical science, especially through his investigation of "vision," that nothing in the universe has any actual being, apart from a universal element, that, wherever it may be posited, can alone be called Mind. Since Berkeley, no philosophical thinker, perhaps, of any significance, anywhere in the world, has questioned the "ideality" of "material things." Even Reid, as the philosopher of "common sense," declared that "No man can conceive any sensation to resemble any known quality of bodies. Nor can any man show, by any good argument, that all our sensations might not have been as they are, though no body, nor quality of body, had ever existed." Hume's comprehension of Scientific idealism was complete to his day, and was completely stated. He said: "'Tis not our body we perceive when we regard our limbs and members, but certain impressions which enter by the senses; so that the ascribing a real and corporeal existence to these impressions, or to their objects, is an act of the mind difficult to explain." The idealism of recent "materialistic" philosophers, such as Herbert Spencer and the school of "Positivists," has been most carefully expressed by John Stuart Mill, in his statement that "Matter is a Permanent Possibility of Sensation." "If" [said Mr. Mill] "I am asked whether I believe in matter, I ask whether the questioner accepts this definition of it. If he does, I believe in matter; and so do all Berkeleians. In any other sense than this I do not."