Appearance is representation... it's an illusion, to a degree. Perception and "existence" are interchangeable terms. "Since substance is measured, and considered "real" according to sensory description; what would the "world of substance" be - without senses? How can an object which depends on the mind for its manifestation in appearance, have an appearance, when no mind is present? The appearance of things are "caused" by the thing-in-itself, which is external to us. It's like this... the thing-in-itself is the DVD, and the mind is the DVD player; the footage "ON" the DVD, not the DVD itself, are the objects - like a "red cup". Now, if there was NO DVD player, no minds, how much of a reality would that "intangible" footage, appearances of objects, be, without any DVD player to interpret the DVD's message? The DVD player, the mind, is pre-hardwired, in regards, on how to "interpret" the data being manifested - the thing-in-itself. "Reality" adheres to the dictates of the perceivers perception(The DVD player) of the thing-in-itself.