"Every impression, external and internal, passions, affections, sensations, pains, and pleasures, are originally on the same footing."[1] (2) The idea of the external existence of our perceptions is obtained neither from the senses, nor from reason, but from the imagination.[2] (3) Belief in this external existence is produced by the force and vivacity of the resembling perceptions, in accordance with Hume's general theory.[3] Inquiry: (1) The only objects of human knowledge are impressions and ideas;[4] the mind has never anything present to it but these perceptions;[5] and, the primary and secondary qualities of objects are on the same basis.[6] (2) All men, and even the animal creation, are carried away by a natural instinct to repose faith in their senses, and to imagine an external universe to exist. And when men follow this blind and powerful instinct of nature, they always suppose the very images, presented by the senses, to be the external objects.[7] (3) Belief always consists in the force and vivacity of an impression or idea.[8] The philosophical hypothesis of the double existence of perceptions and objects is discussed in the Inquiry, at least incidentally, and rejected as in the Treatise.[9] The most probable reasons for the omissions in the later work were: (1) Hume's desire to present the main features of his system of philosophy in a brief and popular form; and (2) the contradiction which he discovered in his fundamental principles shortly after publishing the Treatise.[10]
I cannot agree with Erdmann[11] when he says that Hume never doubted what was regarded beyond everything else as doubtful by the skeptics of antiquity, viz., the existence of what we perceive. Hume never doubted the existence of our perceptions, or impressions and ideas, as states of consciousness, but he denies the possibility of ever obtaining any knowledge of anything different from such perceptions. It seems to me that this is precisely the position of the ancient
