"...one never manages to determine the instant when a stimulus once seen is no longer seen. There occurs here an indeterminate vision, a vision of something or other, and, to take the extreme case, what is behind my back is not without some element of visual presence. The two straight lines in Müller-Lyer's optical illusion are neither of equal nor unequal length; it is only in the objective world that this question arises. The visual field is that strange zone in which contradictory notions jostle each other because the objects — the straight lines of the Müller-Lyer — are not, in that field, assigned to the realm of being, in which a comparison would be possible, but each is taken in its private context as if it did not belong to the same universe as the other. Psychologists have for a long time taken great care to overlook these phenomena. In the world taken in itself everything is determined. There are many unclear sights, as for example a landscape on a misty day, but then we always say that no real landscape is in itself unclear. It is only so for us. The object, psychologists would assert, is never ambiguous, but becomes so only through our inattention. The bounds of the visual field are not themselves variable. and there is a moment when the approaching object begins absolutely to be seen, but we do not 'notice' it. But the notion of attention...is supported by no evidence provided by consciousness. It is no more than an auxiliary hypothesis, evolved to save the prejudice in favour of an objective world. We must recognise the indeterminate as a positive phenomenon. It is in this atmosphere that quality arises. Its meaning is an equivocal meaning; we are concerned with expressive value rather than logical signification. The determinate quality by which empiricism tried to define sensation is an object, not an element, of consciousness, indeed it is the very lately developed object of scientific consciousness. For these two reasons, it conceals rather than reveals subjectivity."
Phenomenology of Perception, by Merleau-Ponty (p. 6-7)
