George Berkeley believed that there are only minds and ideas. The existence of minds (or spirits or souls), Berkeley contended, consists in perceiving whereas the existence of ideas (including sensations) consists in being perceived. Minds, which are the only substances, are active, and ideas are passive. The existence of physical objects consists in their being perceived. This is so because such objects consist of their qualities, and qualities are sensations. Thus Berkeley endorsed the idealist view that the physical world is kept in existence by being perceived. It depends upon the mind and cannot exist apart from perception. Consequently there is no need to presuppose material substance. Indeed, the very concept of material substance is incoherent. God is the source of our sensations. Hence we are in intimate contact with God, and we ought therefore always to be assured of God's existence and to...