These preliminary remarks are, perhaps, enough to show the nature of _the problem that attention sets to a scientific psychology_. We shall be concerned with sensory vividness; we have to find out under what circumstances a sensation or image becomes vivid, and under what circumstances it becomes obscure; we have to trace the pattern of attention in greater detail and with more accuracy; we have to ask how many sensations may be vivid at the same time, and how long they remain vivid; and so on. We must keep the common-sense view always in mind, so that the scientific alternative stands out clearly and distinctly against it; and we must take scientific account of all that common sense lays down. § 20. =The Development of Attention.=—If we consider a large number of cases of attention, we find that they fall into three great groups; and each one of these groups seems to represent a stage in the development of mind at large, a level of mental evolution. We speak accordingly of =primary=, of =secondary=, and of =derived primary attention=. Let us consider them in order. (1) _Primary attention._—There are certain classes of stimuli that force attention upon us; they take us by storm, and we can offer no resistance; when they appear, we must attend, whatever our preoccupation may be. _Intensive_ stimuli belong to this class: very loud sounds, very bright lights, strong tastes and smells, severe pressures, extreme temperatures, intense pains, one and all take possession of us, dominate us in their own right. A stimulus that is often _repeated_ is also likely to attract the attention, even if at first it went unremarked. _Sudden_ stimuli, and sudden changes of stimulus, have the same effect. So with _movement_: the animal or bird that crosses the landscape, the melody that rises and falls to a steady accompaniment, the insect that crawls over our hand as we lie on the grass, all alike constrain our attention. A _novel_ stimulus has the same power; it stands alone and unrelated; it startles or arrests us. Here then is a fairly long list—high intensity, repetition, suddenness, movement, novelty—of controls to which the human organism is subject. Let any one of them come into play, and _the corresponding sensation is made vivid_, shoots to the focus, engrosses us. We may very quickly shake off the control, and return to the business that it interrupted; but we cannot altogether escape it. The irresistible appeal of these various modes of stimulation shows us attention at its first developmental level.