[Footnote 1: Against the Berkeleyan denial of abstract notions the popular philosopher, Joh. Jak. Engel, directed an essay, _Ueber die Realität allgemeiner Begriffe_ (Engel's _Schriften_, vol. x.), to which attention has been called by O. Liebmann, _Analysis tier Wirklichkeit_, 2d ed., p. 473.] The "materialistic" hypothesis--so Berkeley terms the assumption that a material world exists apart from perceiving mind, and independently of being perceived--is, first, unnecessary, for the facts which it is to explain can be explained as well, or even better, without it; and, second, false, since it is a contradiction to suppose that an object can exist unperceived, and that a sensation or idea is the copy of anything itself not a sensation or idea. Ideas are the only objects of the understanding. Sensible qualities (white, sweet) are subjective states of the soul; sense objects (sugar), sensation-complexes. If sensations need a substantial support, this is the soul which perceives them, not an external thing which can neither perceive nor be perceived. Single ideas, and those combined into objects, can exist nowhere else than in the mind; the being of sense objects consists in their being perceived (_esse est percipi_). I see light and feel heat, and combine these sensations of sight and touch into the substance fire, because I know from experience that they constantly accompany and suggest each other.[1] The assumption of an "object" apart from the idea is as useless as its existence would be. Why should God create a world of real things without the mind, when these can neither enter into the mind, nor (because unperceived) be copied by its ideas, nor (because they themselves lack perception and power) produce ideas in it? Ideas signify nothing but themselves, _i. e_., affections of the subject. [Footnote 1: The fire that I see is not the cause of the pain which I experience in approaching it, but the visual image of the flame is only a sign which warns me not to go too near. If I look through a microscope I see a different object from the one perceived with the naked eye. Two persons never see the same object, they merely have like sensations.]