As our mental fields succeed one another, each has its centre of interest, around which the objects of which we are less and less attentively conscious fade to a margin so faint that its limits are unassignable. Some fields are narrow fields and some are wide fields. Usually when we have a wide field we rejoice, for we then see masses of truth together, and often get glimpses of relations which we divine rather than see, for they shoot beyond the field into still remoter regions of objectivity, regions which we seem rather to be about to perceive than to perceive actually. At other times, of drowsiness, illness, or fatigue, our fields may narrow almost to a point, and we find ourselves correspondingly oppressed and contracted.
Different individuals present constitutional differences in this matter of width of field. Your great organizing geniuses are men with habitually vast fields of mental vision, in which a whole programme of future operations will appear dotted out at once, the rays shooting far ahead into definite directions of advance. In common people there is never this magnificent inclusive view of a topic. They stumble along, feeling their way, as it were, from point to point, and often stop entirely. In certain diseased conditions consciousness is a mere spark, without memory of the past or thought of the future, and with the present narrowed down to some one simple emotion or sensation of the body.
The important fact which this ‘field’ formula commemorates is the indetermination of the margin. Inattentively realized as is the matter which the margin contains, it is nevertheless there, and helps both to guide our behavior and to determine the next movement of our attention. It lies around us like a ‘magnetic field’, inside of which our centre of energy turns like a compass-needle, as the present phase of consciousness alters into its successor. Our whole past store of memories floats beyond this margin, ready at a touch to come in; and the entire mass of residual powers, impulses, and knowledges that constitute our empirical self stretches continuously beyond it. So vaguely drawn are the outlines between what is actual and what is only potential at any moment of our conscious life, that it is always hard to say of certain mental elements whether we are conscious of them or not.
—William James (Varieties of Religious Experience, Lecture 10, pp. 214f.)