Everyone knows what attention is. It is the taking possession by the mind, in clear and vivid form, of one out of what seem several simultaneously possible objects or trains of thought. Focalization, concentration, of consciousness are of its essence.
William James, Principles of Psychology
Those who lay stress on the unity of mind regard it as almost evident a priori, that but one concept can occupy the focus of attention at a time... Attention, like the lens of the eye, is now [ie, first] accommodated to act as an instrument of near focus, high magnification, but limited aperture, and again [then] as one of distant focus, small magnifying-power, but wide range
Can the Mind attend to two things at once? Science: July 18, 1887