Plato's Cave: Alternative Views

Alternative Views

What are the alternatives to the idea that the world you see around you is an internal percept?

The Realist View

The most straightforward alternative is to say that the world you see around you is the real world, and that the head that you know as your own, is your real head. This is the most familiar and obvious interpretion, known in philosophy as Realism, i.e. you believe what you see.

The problem with this view is captured by Plato's parable. If the room out there is the real room, then it should not be directly visible to you, but only through the mediation of your senses. This objection is based on the assumption that the brain is a physical mechanism that receives input from the senses, and that perception is a manifestation of the activity of the brain, i.e. that a percept is a pattern of neural activity within the brain representing external objects and events. When you look at the room around you, where is that pattern of neural activity? Is it within the head that you know as your own? Or is it "out there" in the world around you? If it is inside your head, then how does the percept get back out of your head?

Furthermore, if the world around you were the real world, why does it reflect the distortions seen in visual illusions, which appear not as an error or distortion in your head, but a distortion in the world around you?

If a percept is a pattern of neural activity in the brain, and you perceive the world around you, then that world must be either the pattern of neural activity itself, or at least the information represented by that pattern of activity. This precludes the possibility that the world you see around you is the real world itself.

Projection Hypothesis

Another possibility is that perception occurs within your brain, but that the percept gets "projected out" and superimposed on the external world. If so, what is the mechanism of this projection? How does it relate to known neurophysiology?

Direct Perception

Gibson [Gibson 1904] was addressing the same question posed by this argument: if the retinal input is so impoverished, how does your percept of the world become so rich? Gibson however chose the opposite conclusion: the world appears so rich in detail because your perception of that world is "direct", i.e. that it occurs directly in the world around you, rather than inside your head. This argument is certainly appealing on subjective grounds, because the world does indeed appear to be outside your head, and contains far more detail than whatever it is that we experience inside the head that we know as our own. The problem with this view is that it suggests that perception somehow bypasses the bottleneck of the senses, and is thus inconsistent with the idea that perception is a computation performed by the brain on the information received from the senses.

The World is a Sufficient Model of Itself

A variation of the direct perception idea is a hypothesis supported by Rodney Brooks [Brooks 1991] and Kevin O'Regan [O'Regan 1992] (among others), that the world is a sufficient model of itself, and can be sampled immediately by visual saccades as if the external world were an internal memory, i.e. a visual saccade is analogous to a memory access from an internal memory, except that that information is stored externally as the world itself. The problem with this view is that the information sampling of the saccade cannot read the three-dimensional information directly from the world, but only through the retinal image which is a much impoverished two-dimensional projection of the external world, and the three-dimensional interpretation must be reconstructed from that two-dimensional projection. This reconstruction operation is not a trivial matter, as it depends on a good deal of contextual information.

The idea that the external world can be accessed as if it were an internal memory only seems intuitively plausible because of the illusion of Plato's Cave, i.e. when we think we are looking at the external world, we really are looking at the internal copy of that world, which is indeed accessing an internal rather than an external memory. If however we could see how the raw input appeared before the processing performed by the visual system, this idea would be patently absurd. Indeed, patients with the condition of visual agnosia do see the world without pre-processing, and cannot make any sense out of that visual input. Visual agnosia is not a problem in the accessing of the external information, for the retinal input comes in just fine. Rather it is a problem of assembling and interpreting that information as a three-dimensional percept which is deficient.

Abstraction

The most common view represented in computational models of vision is that perception is a process of abstraction from the visual input, i.e. that the visual system extracts features such as edges and surfaces from the retinal image, and passes them on to higher level processing stages which extract ever more abstract and high level features from the lower level representations, resulting in an abstract compressed representation of the external world in some internal symbolic code. The problem with this view is that it completely ignores the most salient manifestation of perception, i.e. the very solid and real-looking world that appears to surround you. This world is clearly not an abstraction, for unlike the retinal image it is fully spatial, and rendered at the highest possible resolution. Indeed it contains far more information than the retinal input upon which it is based. Where in the hierarchy of abstraction is this percept represented?

Any Other Ideas?

If I have missed any alternatives which resolve this dilemma, please send me email and let me know, and I will include it here.

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