Like most philosophers of the period, Berkeley seems to assume that touch provides immediate access to the world. Visual ideas of an object, on the other hand, vary with one's distance from the object. As one approaches a tower one judges to be about a mile away, "the appearance alters, and from being obscure, small, and faint, grows clear, large, and vigorous" (NTV §44). The tower is taken to be of a determinate size and shape, but the visual appearance continually changes. How can that be? Berkeley claims that visual ideas are merely signs of tactile ideas. There is no resemblance between visual and tactile ideas. Their relationship is like that between words and their meanings. If one hears a noun, one thinks of an object it denotes. Similarly, if one sees an object, one thinks of a corresponding idea of touch, which Berkeley deems the secondary (mediate) object of sight. In both cases, there are no necessary connections between the ideas. The associative connection is based on experience (NTV §51; cf. TVV §40, Alciphron, Dialogue 4).