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I am not sure if this question should be asked in the Philosophy or Neuroscience forum as both domains are relevant to this inquiry. Will have a go anyway.

One of the defining characteristics of CBS are the hallucinations that appear as a result of some impairment to vision. They range from simple to complex. Examples are given from geometrical shapes to people and landscapes and many have studied and provided commentary on this and similar phenomenon for many years. When thinking about them (and imaginal phenomena in general) I recall the ancients and find just how relevant and valuable their inquiries are today. Let us consider the following as an example:

On "The Enneads of Plotinus, Volume 1: A Commentary" By Paul Kalligas page 557 III 6. 'On impassibility' he states:

"Matter is, at heart, a place where images are produced and thus itself may be characterized as an image [eidolon] that gives rise to fleeting and deceptive illusions but that can never be pinpointed or apprehended on its own. At the risk of simplification we could say that it is the indeterminate and imperceptible constituent of corporeal entities considered as images. For while the configuring elements of an image, which in themselves are subject to our powers of perception, may all be referred, in one way or another, to the intelligible models they figure, the nature of the image as such cannot be explained solely by the specification of its relation to these models. Something further is required: a declaration of its otherness with respect to these, in order that the dependent character of its nature as an image may be rendered perspicuous. The 'non-intelligible' element, which makes images be images and their subsistence be purely phenomenal --- an "apparition" [phantasma] or "ghostly image of bulk" [phantasma onkou]--- is matter. On this see also the highly pertinent, if somewhat cryptic, observations of Gerson 1994, 112. It is, in other words, a necessary condition for the possibility of images being produced: for it does not itself, literally, produce them. It merely has a predisposition to produce them and a "propensity" or "aspiration" to give them subsistence so that in this sense it "longs for that which is destructive [sc., of itself]" (Oregetai tou phthartikou: cf. Arist. Metaph. N 4, 1092a2 and my discussion of this passage in my introduction to I 8) notwithstanding that in reality it cannot incur anything and consequently cannot suffer  destruction. It may be perceived indirectly because there appear fleetingly upon it all sorts of conflicting and mutually contradictory qualities such as those that Plato invoked in his unwritten doctrines in order to designate his own material ontological principle. On this see Arist. PH I 4, 187aI6-18, Metaph. A 6, 987b2o; and Hermod. Fr. 7 (apud Dercyllides Porph. 146F apud Simpl. in Ph 247.34-248.8): "Plato, assuming matter to be ranged with the unlimited and indefinite, clarifies its nature on the basis of these as belonging to those things which admit the more-and-less of which the great-and-small are one."<

In "A Vocabulary of the Philosophical Sciences

(Including the Vocabulary of Philosophy, Mental, Moral and Metaphysical, by William Fleming, from the 2d Ed., 1860: and the 3d, 1876, Ed. by Henry Calderwood, LL. D.)" By Charles Porterfield Krauth, William Fleming, Henry Calderwood · 1877 it mentions Hobbes suggesting that:

"Space is the mental image (phantasma) of a thing existent as existent, that is, no other accident of that thing being considered, except that it appears exterior to the person having the image."<

What is known is that a transformation (or conversion if you will) is occurring via transduction, in the case of CBS the impairment thereof with the brain accommodating for same, but what is brought into question is the topography (locus of manifestation) of visual experiences [e.g. "where is it that I go when I dream?"].

https://2dcurves.com/3d/3dh.html

The horopter seems to be the topos for experiences like optical illusions, hypnogogic imagery or eidetikos and the like. As though they were projected from the fovea and into the vieth-muller circle, against Panum's fusion area and unto the zones of disparity; like any other image that is projected onto a screen. They surely cannot be plastered, so to speak, on the lens or cornea of the eye.

This is an utterly doubtful situation either way...but why does it appear as though they are "exterior" as Hobbes describes? Is the "apparition" or "phantasma" matter? If so then of what kind? If not then why?

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Is the "apparition" or "phantasma" matter?

No, of course not. It's a hallucination. It has material existence only among the neurons in the head of the sufferer.

Suppose for comparison that you have a webcam set up, pointed at a potted plant, and sending the visual information of the plant to a computer, so that the image of the plant shows on your computer screen. This is analogous to a properly functioning human vision system. Now suppose that someone has hacked your webcam so that instead of a plant, the webcam sends the image of an elephant to your screen. This is analogous to Charles Bonnet syndrome; your computer is "hallucinating." Does it mean that the elephant shown has material existence? No. There isn't any elephant, there's just a plant. The elephant exists only within the electronic signals processed by the camera and computer.

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  • If I follow your logic we would say that despite their material existence in neurons they are not made of any matter in particular. This would appear to violate the law of non-contradiction but I would assume that you would contest because you admit that hallucinations and neurons are not the same phenomenon. We must then ask what we mean by "hallucination" in order to avoid conflating the map with the territory should we not? The webcam scenario though analogous is not exactly comparable. As "mental images" are not images that can emerge from a computer.
    – Somnis
    Mar 10, 2021 at 4:05
  • @TriSatNava ??? The neurons are made of matter, sure. Doesn't change the fact that the hallucinated image isn't really there. We may compare it to a person looking at a chair. If the chair is really there, then two things have material existence: the chair, and the patterns of neural activation in the person's brain. If the person is instead hallucinating the chair, then only the patterns of neural activation have material existence, but not the actual chair. With a hallucination you have one thing, instead of two.
    – causative
    Mar 10, 2021 at 4:13
  • What kind of a thing can appear as though it is occupying a spatial coordinate and yet is said to be absent? If I concede to your point and suggest that the imaginary chair is bereft of all material then what we have left is an image of a material thing. Now it has become a dichotomy between an image and the matter that it is said to have a relation to.
    – Somnis
    Mar 10, 2021 at 4:28
  • @TriSatNava Are you equally confounded when you look at a picture of a tropical beach in the newspaper, and wonder whether there is a real beach right there in your kitchen? Do photorealistic paintings present to you the same problem? Many things seem-as-if and yet are-not. If I make a bird call that truly seems-as-if a real bird is calling, would you say there is a real bird there?
    – causative
    Mar 10, 2021 at 4:35
  • I would say no to the first two scenarios but it would depend on how good you are at bird-calling for the third. However, the examples that you offer, though they can be considered as food for thought, are false equivocations.
    – Somnis
    Mar 10, 2021 at 4:42

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