What then is the difference between understanding a situation and the more usual reactions to it? It is a difference in the degree of organisation of the impulses which it arouses. It is the difference between a systematised complex response, or ordered sequence of responses, and a welter of responses. We must not take ‘understanding’ in too specialised a sense here, or we shall overlook the immense importance of this difference in determining revival. We are accustomed to make an artificial distinction between intellectual or theoretical and non-intellectual or emotional mental activities. To understand a situation in the sense here intended is not necessarily to reflect upon it, to inquire into its principles and consciously distinguish its characters, but to respond to it as a whole, in a coherent way which allows its parts their due share and their proper independence in the response. Experience which has this organised character, it is reasonable to suppose, has more chance of revival, is more available as a whole and in parts, than more confused experience. Contrast the behaviour of the sleepy and the fully awake, of the normal man with the lightly and the more deeply anæsthetised patient, of the starved or fevered with the healthy. To describe these differences in neural potency, and to mark the degree of physiological efficiency, Dr Head has recently[57] suggested the term _vigilance_, a useful addition to our symbolic machinery. In a high state of vigilance the nervous system reacts to stimuli with highly adapted, discriminating, and ordered responses; in a lowered state of vigilance the responses are less discriminating, less delicately adapted. Whether we are considering the decerebrate preparation or the intact poet, the simplest automatisms or the most highly conscious acts, what happens in a given stimulus situation varies with the vigilance of the appropriate portion of the nervous system. The point as regards revival can be put conveniently by saying that experiences of high vigilance are the most likely to be available. The degree of vigilance of the individual at the moment at which revival is attempted is, of course, equally but more evidently an important factor. The answer then, at least in part, to the problem of how the poet’s experience is more than usually available to him is that it is, as he undergoes it, more than usually organised through his more than usual vigilance. Connections become established for him which in the ordinary mind, much more rigid and exclusive in its play of impulses, are never effected, and it is through these original connections that so much more of his past comes to be freely revivable for him at need.