It's not even a philosophical issue; it's a scientific issue. And the neuroscience shows us that visual percepts exist when there is neural activity in the visual cortex.

Visual percepts evoked with an intracortical 96-channel microelectrode array inserted in human occipital cortex.

Your explanation of what causes variations in colour perception is not relevant to the claim I am making.

I see white and gold when I look at the photo of the dress. The nouns "white" and "gold" in the preceding sentence do not refer to the screen's "disposition" to emit the wavelengths of light typically associated with white and gold; they refer to the types of mental percepts that I have and that those who see black and blue don't have.

Two people looking at the same photo on the same screen can see different colours. See the dress.

I see white and gold, my colleague in the same room looking at the same screen sees black and blue.

The nouns "white", "gold", "black", and "blue" in the preceding sentence are referring to percepts, not mind-independent properties of the screen or the light. This is the ordinary use of such nouns.

See also variations in colour perception.

Maybe quote the rest of the sentence:

When I look at the photo of the dress and describe its colours as white and gold, the words “white” and “gold” are referring to colour percepts, not the pixels on the screen emitting certain wavelengths of light

Do you agree or disagree with this?

When looking at the photo of the dress, some see white and gold, some black and blue. This is a fact. What are the words "white", "gold", "black", and "blue" referring to in that sentence?

I say mental percepts.

Six months later, Michale is still here to argue that he is most probably a Boltzmann brain — Banno

I'm not arguing that here. I'm simply responding to the question asking if brains can generate experiences in the vacuum of space. The Boltzmann brain thought experiment shows that such a scenario is both coherent and consistent with current scientific theories.

When one has an experience, it is an experience of something. When there is no "something", it's an hallucination. — Banno

You seem to be using "experience" to mean "veridical experience". You're welcome to, but that's not what is meant when discussing Boltzmann brains.

As an example, taken from Wikipedia:

Experience refers to conscious events in general, more specifically to perceptions, or to the practical knowledge and familiarity that is produced by these processes. Understood as a conscious event in the widest sense, experience involves a subject to which various items are presented. In this sense, seeing a yellow bird on a branch presents the subject with the objects "bird" and "branch", the relation between them and the property "yellow". Unreal items may be included as well, which happens when experiencing hallucinations or dreams.

When I talk about experience I mean any occurrence of visual or auditory or other percepts, produced by activity in the visual or auditory or other cortexes, regardless of what caused it. The experience is veridical if it is caused by the appropriate external stimulus, a dream if asleep, or an hallucination or illusion otherwise.

The colours in the photograph are susceptible to blend and interfere with changing light conditions on different screens and environments where the photo is displayed. Basically we don't just see the colours of the dress, but a blend of its colours with the colours from different environments or screens, and that's why different observers tend to see different colours. — jkop

We look at the same distal object (the pixels on the screen), our eyes react to the same proximal stimulus (the light), and yet we see different colours. Therefore, the noun "colours" in the preceding sentence is not referring to some property of the distal object or some property of the proximal stimulus; it is referring to the type of mental percepts that differ between individuals.

And the way we're using the noun "colours" in that initial sentence is the ordinary use of the word.

That's plainly false. Red paint really reflects wavelengths of 700 nm, and to experience it as red is to have a veridical experience of it (unlike experiencing 700 nm as gray (if colorblind) or as any colour, sound, smell etc. (if hallucinating). — jkop

You're putting the cart before the horse. Light with a wavelength of 700nm ordinarily causes red colour percepts in most humans, and so we use the adjective "red" to describe objects which reflect light with a wavelength of 700nm.

But as a noun, the word "red" ordinarily refers to that type of percept.

Historically, before we knew what these percepts were, we mistook them for being mind-independent properties of objects. This is the naive realist view. We know better now.

So you're denying the existence of dreams, hallucinations, synaesthesia, variations in colour perception, and basically the entire neuroscience of perception.

Well, you're welcome to, but you're wrong.

There is no practical reason to refer to "mental percepts" at all, or for that matter — Richard B

Yes there is; to make sense of dreams, hallucinations, synesthesia, variations in colour perception (e.g. the colours of the dress are white and gold to some and black and blue for others), and visual cortical prostheses.

And these mental percepts exist in ordinary waking experiences to. They are what we ordinarily refer to when we use colour nouns, knowingly or not. An object reflecting various wavelengths of light is just the ordinary cause, nothing more.

How do we perceive this propensity? — Hanover

How do we perceive a fire’s propensity to cause pain? By putting our hand in the fire and being hurt. In the case of colour, we look at the pen and see red.

Do we just assume our perceptions are externally caused? — Hanover

I think it’s a little more than an assumption. Perhaps it’s the most rationally justified explanation.

Since all perceptions are subjective responses, you can't claim any property to exist objectively, except to just say the perceptions must be being elicited by something. — Hanover

We can claim anything we like. Some are true, some are false, and some may be more justified than others.

I think it’s justified to claim that mind-independent chairs exist but that mind-independent pain doesn’t, and most would agree. Clearly there’s just less of a consensus regarding whether or not colours are more like chairs or more like pain. I think modern physics and the neuroscience of perception shows them to be more like pain.

That is, an atom has no particular shape, size or color. It just makes me see what I think to be a chair. — Hanover

That would certainly be the Kantian view, and I’m sympathetic. But I’m not arguing for anything that extreme. I’m only arguing that colours, like pain, are a mental percept.

Yet, I can see black objects. I can pick out an object that is black from other objects that are colored. Why can't we say it lacks the property of color? What makes less sense is to say I pick out a black object because it has no mental percepts. I pick it out because it was black. — Richard B

I have repeatedly drawn a distinction between the adjective “red” and the noun “red”.

We can use the adjective “red” to describe a mind-independent pen that has properties that are the cause of red colour percepts. But the noun “red” refers to that colour percept, not a mind-independent property of the pen.

If you don't distinguish between experience (i.e. event in your brain) and colour (i.e. object of the experience), then you can't distinguish between veridical experiences and hallucinations. How could any animal have survived on this planet if they were only hallucinating and never saw objects and states of affairs? Arguments from illusion or hallucination suck. — jkop

In the case of colour there is no such thing as veridical. It’s not “correct” that light with a wavelength of 700nm causes red colour percepts, such that if a different organism with different eyes and brain sees a different colour in response to 700nm light then they are seeing the “wrong” colour.

I didn't say that. I said that the pigment and the light have the disposition to systematically cause the experience of colour. This means that the colour experience arises when an animal that has the ability sees the pigment or light, while the colour is a property of the pigment or light in the form of a disposition. — jkop

We can use colour terms however we like, but when we ordinarily use them we are referring to colour percepts, not an object’s disposition to reflect a certain wavelength of light.

When I look at the photo of the dress and describe its colours as white and gold, the words “white” and “gold” are referring to colour percepts, not the pixels on the screen emitting certain wavelengths of light, and when someone else looks at that same photo and describes its colours as black and blue, the words “black” and “blue” are referring to colour percepts, not the pixels on the screen emitting certain wavelengths of light.

If the words “white”, “gold”, “black”, and “blue” were referring to the pixels on the screen then the very claim that some see white and gold and others see black and blue (when looking at the same screen) would make no sense at all. Yet it is both coherent and factual.

Things are black when they absorb all (visible) frequencies of light, and so do not re-emit any (visible) frequency of light.

As such there is no (visible) light to stimulate the rods and cones in our eyes, and so the V4 neurons are not fired, and so no colour percepts are produced.

It’s certainly not the case that black is some mind-independent property of objects that is seen by the absence of (visible) light. That just makes no sense at all.

the colour that you experience exists regardless of being experienced — jkop

The surface layer of atoms with a configuration of electrons that absorbs certain wavelengths of light and re-emits others exists regardless of being experienced.

But this isn’t colour. Colour is the mental percept created by neural activity in the visual cortex. That is how coloured dreams, coloured hallucinations, synesthesia, and variations in colour perception are possible.

Referring to mind-independent objects as having colours is a relic of naive colour realism, the mistaken view that either confuses colour percepts for being mind-independent properties or falsely believes that, in the veridical case, colour percepts resemble mind-independent properties. Our modern scientific understanding of the world and perception has corrected us of this misunderstanding.

It is just "physical system capable of producing consciousness." — Count Timothy von Icarus

Such as a brain:

The scenario initially involved only a single brain with false memories, but physicist Sean M. Carroll pointed out that, in a fluctuating universe, the scenario works just as well with entire bodies, even entire galaxies.

…

… human brains are vastly more likely to arise from random fluctuation …

…

Boltzmann-style thought experiments generally focus on structures like human brains that are presumably self-aware observers.

Although this seems to be moving beyond the relevant point, which is that colour percepts are the product of neural activity in the visual cortex. This neural activity can be caused by optical stimulation by light, e.g. when awake, or by other things, e.g. when hallucinating or dreaming. We don’t need to posit some additional mind-independent colour; we already have a parsimonious account of colour perception consistent with the scientific evidence.

Are you under the impression that Boltzmann brains actually exist? — Count Timothy von Icarus

No, I’m saying that they are a coherent concept and consistent with current scientific understanding.

Weird that a chameleon would change my mental phenomena(the color of the chameleon) and result in blending into its surroundings which are not my mental phenomena. — creativesoul

They change the way their skin reflects light. Different wavelengths of light elicit different colour percepts.

You asked if brains can generate experiences in the vacuum of space. Boltzmann brains are formed in the vacuum of space, and “in Boltzmann brain scenarios … Boltzmann observers who have the same series of experiences as me … vastly outnumber normal observers.”

So, yes, apparently brains can generate experiences in the vacuum of space. All that is required is the appropriate neural activity, regardless of what causes and maintains it.

Does a brain generate any experience on the ocean floor? On the surface of a star? In the void of space? — Count Timothy von Icarus

You should read up on Boltzmann brains.

But why? — Banno

We talk about various things in the world. Some of those things are mental phenomena, some aren't. Some of those things are trees, some aren't. I don't understand the difficulty you're having.

Your question is like asking why the noun “dog” picks out an animal and not a planet.

In addition to what? — Banno

As in, among the various types of mental phenomena, colour is one such type.

Why shouldn't a red pen simply be a pen that reflects light at various wavelengths and various intensities? — Banno

You can use the adjective "red" to mean "reflects light with a wavelength of 700nm" if you like, but when we ordinarily talk about colours (note that I'm now using a noun) – particularly when we ask if colours are mind-independent and discuss the fact that some see white and gold and others black and blue – we are talking about colour percepts, knowingly or not.

And there’s certainly no “resemblance” between a red colour percept and a surface layer of atoms that reflects light with a wavelength of 700nm. The relationship between the two is merely causal, and that the latter causes the former is a contingent fact about human biology. Different organisms with different eyes and brains can have different colour percepts in response to 700nm light. The photo of the dress is proof of that even within humans.

How come "pen" picks out a mind-independent object, and not just whatever has the causal role in eliciting a particular type of mental percept. Doesn't the noun "pen" refer to this type of mental percept? — Banno

Because some words pick out mental phenomena and some words don't? Even the naive realist must accept this; words like "mind", "consciousness", "pain", "pleasure", "beliefs", "disgust", and so on pick out mental phenomena.

I am simply explaining that colours are also a type of mental phenomena, not a type of mind-independent property of pens. As I said to Richard B above, the pen just reflects light at various wavelengths and various intensities and then our eyes and brain respond in deterministic ways to this electromagnetic energy, activating the visual cortex and producing visual percepts (including colour percepts).

The naive view that projects these colour percepts onto mind-independent objects, or that thinks that some mind-independent property "resembles" these colour percepts, is mistaken.

The noun "pen" refers to a mind-independent object. The adjective "red" describes this mind-independent object's causal role in eliciting a particular type of mental percept. The noun "red" refers to this type of mental percept.

I think I've been really clear on this.

In this example, are the contact lenses causing new mental phenomena? — Richard B

Yes.

Or, are they just allowing us to see the colors the fruit had all the time. — Richard B

The fruit just reflects light at various wavelengths and various intensities; that's it. Our eyes and brain then respond in deterministic ways to this electromagnetic energy, activating the visual cortex and producing visual percepts (including colour percepts).

For example, a person took a hallucinogen which put the brain in a particular physical state, and thus caused the hallucination. Is this not enough to explain what is happening without appeal to mental phenomena? — Richard B

I haven't said that mental phenomena aren't just particular brain states. I'm not necessarily arguing for any kind of dualism. I'm leaving that open. Maybe pain just is the firing of C-fibers, as Churchland argues. Maybe colours just are the firing of V4 neurons.

Regardless of what mental phenomena are, pain and colours are mental phenomena; they are not mind-independent properties of fire.

on your account we are talking not about the red pen but each of our own solipsistic percept-of-red-pens — Banno

No I'm not.

So on your account, when we agree that the pen is red, we are talking about quite different things - the percept-in-my-mind and the percept-in-your-mind. — Banno

No, because we're using "red" as an adjective to describe the mind-independent pen.

We all agree that this pen is red (causes red mental percepts), just as we all agree that stubbing one's toe is painful (causes pain mental percepts).

But "red" and "pain" as nouns refer to mental percepts, not to some mind-independent property of pens.

Of course, this is generally presented as the squares themselves being "the same color." You can confirm this by looking at the hex codes of the pixels that make them up.However, on an account where grayness, shade, hue, brightness, etc. are all purely internal and "exist only as we experience them," it seems hard to explain the illusion. If the shades of gray appear different, and color just is "how things appear to us," in what sense are the two squares the "same color gray?" It seems that their color should rather change with their context. — Count Timothy von Icarus

They emit the same wavelength of light, but produce different colour percepts. It's the same principle involved with the photo of the dress.

Colour terms like "grey", "black", "blue", "white", and "gold" are then used in at least two different ways, either referring to the fact that they emit the same frequency of light ("the squares are the same shade of grey") or to the fact that they produce different colour percepts ("I see white and gold; she sees black and blue").

The use of such terms to refer to the fact that they emit the same frequency of light is something of a fiction, premised on the misguided naive realist view that treats colour percepts as being mind-independent properties (or, at the very least, the misguided view that colour percepts "resemble" in some sense mind-independent properties).

You said that movies cannot be funny, the lemons are not sour, and that apples cannot be red. — Count Timothy von Icarus

I clarified what I meant in that aforementioned post.

The nouns "sour" and "red" refer to mental percepts – those things that also exist when we dream, hallucinate, have synesthesia, and so on, and which explain differences in perception.

The adjectives "sour" and "red" when predicated of lemons and apples describe the fact that they are causally responsible for the associated mental percept in ordinary waking situations.

So apples are red and lemons are sour.

But this doesn't mean that redness and sourness are mind-independent properties of apples and lemons as the naive realist believes.

Pace your appeal to "science," the science of perception does not exclude lemons from an explanation of why lemons taste sour or apples from the experience of seeing a red apple. These objects are involved in these perceptions; the perceptions would not exist without the objects. — Count Timothy von Icarus

Their involvement is causal, nothing more. The window wouldn't have broken if the football had not been kicked through it, but the broken window is not a property of the football. And the sour-taste mental percept would not be present if I had not eaten the lemon, but the sour-taste mental percept is not a property of the lemon (and nor does it resemble any of the lemon's properties).

How exactly does your account even allow for the coherence of dreams, hallucinations, synesthesia, and difference in colour perception, let alone their facticity?

I think you're misunderstanding my position. Here and here set it out clearly.

I always have. I just deny that colours are something other than mental percepts, just as I deny that pain is something other than a mental percept.

There are mind-independent properties that are causally responsible for colour percepts in the ordinary waking case, e.g. having a surface layer of atoms that reflects light with a wavelength of 700nm, but the colour percept and this surface layer of atoms are distinct, dissimilar, things. And as nouns, the words "red" and "colour" ordinarily refer to these percepts. This is what allows variations in colour perception to be a coherent concept.

As adjectives, the words "red" and "coloured" can describe distal objects, but this just means that they are causally responsible for the related colour percepts in the ordinary waking case.

This view contrasts with the naive colour realist who believes that as nouns the words "red" and "colour" refer to some mind-independent property that resembles the colour percept, and who often denies the existence of the colour percept entirely. This naive colour realism is inconsistent with physics and the neuroscience of perception.

SO are you saying you can have my "mental percepts"? — Banno

No, I'm saying that I can talk about them, just as I can talk about your thoughts even though I can't think them.

Maybe this will make my position clearer:

1. Colour percepts exist. They are what constitute (coloured) dreams and hallucinations.
2. These colour percepts also exist when awake and not hallucinating, e.g. when there is neural activity in the visual cortex in response to optical simulation by light.
3. When we ordinarily talk about colours we are, knowingly or not, referring to these colour percepts.

... if "red" is only a mental percept, then when you say “red” it refers to your mental percept, but when I say "red" it refers to my a mental percept. — Banno

How do you infer that? Pain is a mental percept, but when I use the word "pain" I am not referring only to my pain, just as when I use the words "thoughts" and "beliefs" I am not referring only to my thoughts and beliefs. I can refer to someone else's red just as easily as I can refer to someone else's pain and thoughts and beliefs.

It's true that I can't see their red or feel their pain or think their thoughts or believe their beliefs, but I can talk about them just fine.

For the - I think seventh or eighth time - the claim is not that being red or sour or smooth is in no part mental, but that it is not exclusively in your mind alone. — Banno

And, yet again, I accept that the apple is red, where "red" is an adjective and "the apple is red" means something like "the apple is causally responsible for a red visual percept".

But as a noun, colours are only mental percepts, because when I use the word "colour" I am referring to a mental percept and a mental percept is only a mental percept.

And being sour is a property of lemons... — Banno

As an adjective, yes, where this means that eating a lemon will elicit a sour-type mental percept.

We don't generally have the "mental percept" of "sour" in the absence of lemons or some other such food. — Banno

Well I do. My dreams and hallucinations aren't only visual and auditory; I smell and taste and feel.

So rather than us having to guess what you think is going on, set it out for us all. — Banno

I have done. Colours, smells, tastes, etc. are types of mental percepts, caused by neural activity in the appropriate cortexes in the brain. When this activity occurs when I'm asleep I'm dreaming. When this activity occurs when I'm awake, but in response to LSD, I'm hallucinating. When this activity occurs when I'm awake, and in response to ordinary external stimulation such as 700nm light reaching my eyes, I'm having a non-hallucinatory waking experience.

The naive view that then projects these mental percepts out into the wider world as mind-independent properties of things is mistaken.

And if I mean ""A sour taste is not only a 'mind-dependent' property of a lemon"? — Banno

Then you're wrong. Because a sour taste is a mental percept, caused by activity in the gustatory cortex in response to stimulation of the tongue by acidic chemicals.

If by "the lemon is sour" you just mean "[tasting] the lemon will cause a sour-type mental percept" then I agree.

But if by "the lemon is sour" you mean "a sour taste is a mind-independent property of the lemon" then I disagree. This is the naive view that is inconsistent with the science of perception.

And here we go again.

The berry is red. The berry is rough. The berry is sour.

These involve the berry. They are not purely mental. — Banno

And again, I addressed this in my distinction between adjectives and nouns above.

"Red" is not a mental property, whatever that might be. It's a colour. — Banno

Colours, like smells and tastes and pain, are mental percepts. They are what occur/exist when we dream and hallucinate – and also when having an ordinary waking experience.

What if ↪Mp202020 had chosen touch - would you be arguing that being smooth was a mental phenomena? — Banno

I address this in my distinction between adjectives and nouns in my post above.

I am pointing out that it is not only mental. — Banno

And I am pointing out that it is, just like pain.

Sour is used to refer to lemons, etc. all the time. — Count Timothy von Icarus

I address this in my distinction between adjectives and nouns in my post above.

Stubbing one's toe is not a "mental phenomena". — Banno

I didn't say it was.

Sure, in your somewhat illicit terms this might be so. What is shown is that being red is not private. That is, that there are red things is a part of our shared world. — Banno

There's ambiguity here with the English grammar.

The words "coloured" and "painful" are adjectives, the words "colour" and "pain" are nouns, and the word "red" is both an adjective and a noun.

As a noun, the words "colour", "pain", and "red" refer to mental percepts. As an adjective the words "coloured", "painful" and "red" can refer to distal objects, if by such use we mean that the distal objects are causally responsible for these mental percepts.

The existence of mental percepts caused by neural activity in the visual cortex in response to optical stimulation by light is a sufficient and well-supported account of colour (and other visual) experience.

The naive view that either denies the existence of mental percepts or mistakes mental percepts for some mind-independent property is wrong.

So what. — Banno

So the overwhelming agreement that some X is Y is not proof that Y is not a mental phenomenon.

Whether Y is "red" or "painful", there can be an overwhelming agreement that X is Y even if Y is a mental phenomenon.

That overwhelmingly folk agree that stubbing one's toe is painful does not show that pain is not a mental phenomenon.

Pain is a mental phenomenon, despite the overwhelming agreement on which things are painful.

There is simply no connection between your premise and your conclusion.