 We can learn more by considering a family of phenomena known collectively as phi, which is in turn more generally classified under the heading of apparent motion. The simplest example is of two intermittently illuminated lights, flashing back-and-forth. We naturally represent it as a single, moving light. This is not mere sameness of intensity, but representation as an enduring thing. Moreover, it is not surprising that we should evolve to have this response, if we are to successfully track game, or danger, through intermittent presentations. Or think of a row of lights, as in a neon sign, with either a single bulb illuminated but its position varying, or a single bulb unilluminated, again with systematically varying position. Or think of a clock dial, with one bulb out, moving around the circle. The content of our experience is as an enduring single thing, having different positions at different times. This is the phi phenomenon. Speed things up (sometimes called magni-phi), and it ceases to look discrete, instead presenting as a continuously moving thing, which enhances the effect.