have excited the brain. The sound is then heard as coming from a certain region in the external world. Thus perception in the mode of causal efficacy discloses that the data in the mode of sense-perception are provided by it. This is the reason why there are such given elements. Every such datum constitutes a link between the two perceptive modes. Each such link, or datum, has a complex ingression into experience, requiring a reference to the two perceptive modes. These sense-data can be conceived as constituting the character of a many-termed relationship between the organisms of the past environment and those of the contemporary world.
6. Localization.
The partial community of structure, whereby the two perceptive modes yield immediate demonstration of a common world, arises from their reference of sense-data, common to both, to localizations, diverse or identical, in a spatio-temporal system common to both. For example, colour is referred to an external space and to the eyes as organs of vision. In so far as we are dealing with one or other of these pure perceptive modes, such reference is direct demonstration; and, as iso-
