Knowledge of One’s Visual Field
The human visual field is limited and known by us to be limited. It is possible to establish its extent and shape by the strategic placement of stimuli: about 180 degrees and roundish (corresponding to the optics of the eye). We normally don’t pay much attention to it, but we can easily be brought to recognize that the visual field is limited and offers a roughly circular portal to the world—you just need to move your hand up and down and from side to side in front of your eyes to appreciate the limited extent of your visual field. But how exactly do you come to know that your field of vision is thus limited? Can you see those limits? Clearly not: you can’t see the border of your visual field—you have no sense datum of that border, no impression of it, no perception of its contours. The limits of your visual field lie outside your visual field. In order to see these limits you would have to see what lies beyond them, which is impossible. You can see the limits of things within your visual field, as well as their shape, as when you see a circular object; but the circularity of your visual field itself is not something that is visible to you. There is no round stimulus that your eye is responding to. The shape and limits of the visual field are not objects in the visual field; they condition what you see but they are not seen. So you don’t know the geometrical properties of your visual field by seeing these properties; it has an unseen geometry. Accordingly, your knowledge of your visual field’s geometry is not based on perception of it—as your knowledge of things within your visual field is based on perception of those things. This is not perception-based knowledge: your eyes are not responding to the extent and shape of the visual field. It is not as if the boundaries of the visual field are marked by a bright blue line that you can see. This is a kind of vanishing phantom boundary, though real enough anatomically and phenomenally.
How else might it be known? Could it be known by introspection? That seems quite wrong: you can introspect your visual experiences, but you can’t introspect the point at which they come to an end–you can’t introspect their disappearance. The limits of the visual field are no more introspectible than they are visible. Maybe you can introspect the experience of a limit, but not the limit of an experience. Your introspective faculty is no more responsive to the limits of the visual field than your visual faculty, even if it is responsive to the experiences that the visual faculty serves up. What would it even be to introspect the border between visual experience and its absence? Have you ever tried to do such a thing? How would you set about it?
Do you know it by inference? Surely not: you are not aware of the limits of your visual field by inferring these limits from the fact that you see some things but not others. It is not that you know you can see what’s in front of you but not behind you, and you then form the hypothesis that your visual field must be limited. You can actually experience the limits by suitable placement of stimuli; it isn’t like inferring that you have an unconscious from what is evident to consciousness. It is not a scientific theory, capable of being disconfirmed. It is a phenomenological fact, though not one that appears as an ingredient in experience—like color or shape or texture. It is not listed as among the primary and secondary qualities, simply because it is not a perceptible quality of anything; it transcends the perceptible (or the introspectible). Yet it is something that we can know immediately—it is part of our lived awareness. It isn’t a matter of guesswork or speculation, like the limits of objective space. It’s a datum—but not a datum of sense. We sense it but we don’t sense it.
So we don’t know the shape and limits of the visual field either by perception or by introspection or by inference (and certainly not a priori, like mathematics). But we do have this knowledge. Therefore not all of human knowledge is based on these methods of knowing. How we do have this knowledge is not an easy question, but it is clear that we don’t have it in any of the traditionally recognized ways. Thus traditional epistemology is incomplete. It is a fact of our sensory psychology that the visual field has certain limits, and we are aware of this fact, but we don’t know it by any of the standard methods. Is it empirical or a posteriori? Those categories seem too crude to capture this kind of knowledge, based as they are on sensory knowledge of familiar types—such as seeing what lies within the visual field. But knowledge of the visual field is sui generis—not sensory exactly, and not introspective either (or “intuitive”). It is a bit like our knowledge of our own existence in not fitting into the traditional categories, but it is not to be assimilated to that kind of knowledge either. Familiar as it is, it fails to conform to standard epistemology. It demands its own specific epistemology.