  As this passage illustrates, Berkeley does not deny the existence of ordinary objects such as stones, trees, books, and apples. On the contrary, as was indicated above, he holds that only an immaterialist account of such objects can avoid skepticism about their existence and nature. What such objects turn out to be, on his account, are bundles or collections of ideas. An apple is a combination of visual ideas (including the sensible qualities of color and visual shape), tangible ideas, ideas of taste, smell,  etc.[12]   The question of what does the combining is a philosophically interesting one which Berkeley does not address in detail. He does make clear that there are two sides to the process of bundling ideas into objects: (1) co-occurrence, an objective fact about what sorts of ideas tend to accompany each other in our experience, and (2) something we do when we decide to single out a set of co-occurring ideas and refer to it with a certain name (NTV 109).