The importance of the concept of vigilance for psychophysiological research.

The variety of meanings nowadays attached to the notion of "vigilance" essentially conceals H. Head's object in view when introducing the term more than half a century ago. It may be supposed that a major reason for the existing confusion is that Head failed to distinguish explicitly between "vigilance" as a force, i.e., a reorganizing power ("vital activity") on the one hand and as a system's state ("state of the nervous system") established by the action of the reorganizing power, on the other. We try to explain that the original intention of "vigilance" is compatible with recently elaborated proposals according to which living systems have to be regarded as organizationally closed. Rather than regarding an organism only alternatively, i.e., either as organizational closed (alive) or open (dead), "vigilance" implies a graduated description according to the state of order which has actually been realized by "vigilance".