Terminological fluidity is one of the banes of philosophy. What follows is an admirable exercise in terminological fixation by the Worthy Opponent. My comments are in blue.
I have been discussing toothbrushes [mirror images] with David but it’s clear we are being held back by semantics. I am not clear what we respectively mean by “reflection” or “appearance”, or of the green colour of these things. So I will try to set out what I mean or understand by the different terms.
“Phenomenal green”, “green as we see it”, “green as it appears to us”, also David’s “sensation of green” I think all mean the same thing, namely what I mean by green(ness), but let me explain what I understand by “green”.
BV: So far, so good.
1) Greenness is a visible quality of certain objects such as leaves, avocados, algae, brussel sprouts, [some] toothbrushes etc.
BV: The point needs to be put more precisely. Green (greenness) is a determinable with a range of corresponding determinates. (See here for the distinction.) The latter are the specific shades of green. The determinable green is arguably not a visible quality; only the lowest determinates are, the infima species.
2) It is extended. I mean that a green patch is composed of green patches, which are in turn composed of further green patches ad infinitum. i.e. The greenness is continuous, or consists of a set of green points.
BV: This is not quite right either. Yes, a visible green patch can be subdivided, but not to infinity, for soon enough we arrive at sub-patches that cannot be seen, and this long before we get to points. A point is dimensionless: it has a location but no extension. And surely it is true that no color-determinate is visible if unextended.
3) Only a surface, i.e. a two dimensional thing can be green. However the surface is extended in 3D space, because each point can be a different distance from me.
BV: This sounds right to me. Visible green is given only two-dimensionally, even if the 'green' thing in the external world is 'green' all the way through.
4) The green quality is mind-independent, for the following reasons. (i) It exists outside me, (ii) it is a quality of the object which is green, and not a quality of me. (iii) I can no longer see it when I shut my eyes, but it is still there.
BV: Here is where the going gets tough. If 'exists outside me' just means 'mind-independent,' then the first reason begs the question, or is circular. If, however, 'exists outside me' means 'appears outside me,' then the visible need not be mind-independent.
As for (ii), what is the object? 'Object' is notoriously ambiguous. The thing in the external world? But then it hasn't been shown that the visible quality is a property of the object. It might just be a property of the phenomenon in Kant's sense which, though empirically real, is transcendentally ideal.
As for (iii), if the visible quality is still there when I close my eyes, then it would have to be part of the thing itself in the external world, right? But that seems to comport none too well with the visible quality's being a phenomenal item.
5) It is inert, namely unlike heat it has no causal power to affect my senses.
BV: Seem right. The seen green has no causal power. But how can the visible two-dimensional phenomenal quality be both causally inert, and yet still be there when I close my eyes, given that the latter implies that the quality is part and parcel of the thing itself in the external world?
Unlike heat? But surely there is phenomenal heat in contradistinction to heat-scientifically-understood. The felt heat of the hot coffee when I take a sip is not the same as the mean molecular kinetic energy of the coffee-water molecules.
6) Thus it is not equivalent to reflectance properties of leaves or algae, which are powers to affect my senses, as far as we know, but greenness is an inert, non-causal quality. The leaf just is green.
BV: Yes. The reflectance properties are dispositional properties, but there is nothing dispositional about the seen green, the phenomenal sense quality (sensory quale). It is wholly occurrent or actual.
7) It follows that greenness cannot be a reflectance property of green objects, although there may be some unknown causal connection between the property and the quality.
BV: Seems so. Seen green cannot be a reflectance property of 'green' things themselves in the external world, things we call 'green' because they have the power to cause in us sensory qualia that are phenomenally green.
If, as science suggests, the green quality ‘out there’ is caused by neural processes, the greenness of “green” objects is an illusion, for it cannot be a quality of the green object. The causation cannot work in reverse. There is no way that a neural process in the brain can change or affect the quality of any object outside the brain.
BV: So when I am outside looking at my green palo verde tree in the backyard I am under an illusion because the tree in nature (in the external world) cannot be phenomenally green: that visible quality cannot be a property of the tree itself. It is conjured up in my brain by neural processes.
Is there not something dubious in the view that our direct sensory perception (in optimal conditions of lighting, etc.) of things like trees is illusory? If the seen green is illusory, then so is the smelt scent of the blossoms (The Sonoran spring is in full swing.) And so on for all the other so-called secondary qualities/properties. Can we keep the illusoriness from spreading to the primary qualities?
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