For example, the Müller-Lyer illusion is interpreted traditionally as exemplifying a measurement error. The perceiver sees difference in length between two lines that are equal to some standard of measure, say, a ruler. … The Establishment is tempted to say that the perceiver … falsely infers from the play of light at the eyes that the two lines are of different lengths when, in fact, they are of the same length.
What must be assumed to give legitimacy to this claim for perceptual error? The following come quickly to mind: (1) Whatever the proper basis of measurement for describing the figure is, it is one and the same as the basis of the measurement device by which the figure is described. (TSRM, p. 279).
So, I have been wrestling with TSRM for a while now, trying to figure out how they mean to explain visual illusions. They suggest that cognitivists make assumption (1) above, but I am having a tough time parsing this. I am not sure what they are attributing to cognitivists. Let me explain my confusion.
Suppose, just for expository convenience, that the proper basis of measurement for describing the Müller-Lyer illusion is length, that is, that the Müller-Lyer illusion should be described in terms of length. Is that one side of the kind of identity claim TSRM are noting?
So, then is length one and the same as "the basis of the measurement device by which the figure is described"? That doesn't make sense to me. How can length be a device? Is there some typo here?
Any help here would be appreciated.
Reading the passage in the paper, it would seem to me that
ReplyDeletea) the 'proper' basis is referring to how you 'should', in fact, be expressing the measurement, and then the assumption is that
b) the measurement device you actually end up using shares that basis
People assume you 'should' work with units of physics, and so interpret the behaviour of people (the measurement device) from that perspective.
Specific units, rather than just 'length', help here I think. So for the illusion, if the 'proper' basis is cm, you should be measuring it with a cm ruler, and not an inch ruler . The proper basis in this case is the metric system, not the Imperial, so your ruler should share that basis.
Then, if you assume that your measurement device (your observer) is working with a cm ruler but is coming up with the wrong number of cms, then you have a perceptual error because you've given priority to cms as your proper basis, without asking whether people do, in fact work in cms.
Andrew, thanks for this conjecture. I had thought of something along these lines, but I dont' see how it all works out.
ReplyDeleteThe core troublesome phrase for me is, of course, "basis of measurement". What is a basis for measurement? Now you suggest that a basis for measurement is a unit of measure, such as a cm or an inch. In support of that interpretation, there is TSRM's later claim on the next page (TSRM, 1981, p. 280): "In reference to the first assumption, the ecological approach could not commit itself uncritically to a conventional and convenient standard of measure". Conventional and convenient standard of measure does soundlike they are talking about, say, cm versus inches.
There are, however, (it seems to me) two reasons threatening to undermine this interpretation of "basis of measurement".
I. In the second assumption TSRM mention, immediately after (1) above, they write, "(2) The perceiver as measurement device quantifies over the same basis as that of the measurement device by which the figure is described. (To not assume this is to assume something like a mismatch between the measurement of oranges in candela/m by a photometer and the measurement of oranges in kilograms by a balance. Nobody would ascribe error to the photometer because its readings did not confirm those of the balance)."
In this passage, "basis" seems to mean something like a physical quantity, such as luminance or weight. So, if we accept the interpretation that a "basis" in (1) describes units of measure, then it seems we have to say that they are equivocating between (1) and (2).
II. It doesn't look like units of measure could be the source of the Muller-Lyer illusion. What is the idea? That we measure the line (a) above in cm, but line (b) in inches, so that we get a higher number for (a) than for (b)? That sounds implausible to me, but more importantly TSRM don't seem to be saying that. Their account in (1) seems to be about, say, scientists decribing the Muller-Lyer in terms of, say, cm, where the human visual system describes it in terms of inches. But, I don't see how that goes.
On point I:
ReplyDeleteI'm pretty sure basis of measurement means the unit system and there's no equivocation. Remember, a physical quantity is actually a measurement on a scale. There's no such thing as 'candela/m' without a device calibrated according to a conventional standard for this system. TSRM are laying out the idea that there is a 'proper' basis for measuring something, and that you assume your observer is operating on that basis (where 'proper basis' = conventions of physics). That seems unproblematic.
On II:
The suggestion is you can measure both lines with a cm ruler and get the same number twice, or measure both lines with a visual system and get two different numbers, because the visual system is not a cm ruler. The classical interpretation is to privilege the ruler and classify the discrepancy as a perceptual error; but this is only fair if the ruler is, indeed, the proper basis for measurement. The ecological analysis of illusions is that the data actually suggest the ruler should not be privileged and more work needs doing to establish what kind of ruler the visual system is.
Let me pursue point I: You take assumption (2) to be about units of a system of measurement (just as with 1). Ok. In (2) they write,
ReplyDeleteI. In the second assumption TSRM mention, immediately after (1) above, they write, "(2) The perceiver as measurement device quantifies over the same basis as that of the measurement device by which the figure is described. (To not assume this is to assume something like a mismatch between the measurement of oranges in candela/m by a photometer and the measurement of oranges in kilograms by a balance. Nobody would ascribe error to the photometer because its readings did not confirm those of the balance)."
Ok. But, if they were just after different unit systems, why wouldn't they choose an example of length measured in cm versus lenght measured in inches? Or weight measured in kilograms and weight measured in pounds? Or luminance measured in candelas/m2 versus luminance measured in footlamberts?
What they did was not just units, but the things measured.
(I want to bracket what is to count as a "proper" basis of measurement until I have a handle on what a basis of measurement is.)
Actually, this seems wrong to me: "a physical quantity is actually a measurement on a scale." There are physical quantities, such as mass, temperature, and length, and they pre-existed all our systems of measurement. They have been around for millions of yeats (at least). By contrast, systems of measurement are human creations, right? So, they have been around for what, a few thousand years.
ReplyDeleteSo it seems to me that physical quantities are not measurements on a scale.
Temperature is entirely a measurement on a scale. There are at least 3 in common use: Celsius, Fahrenheit and Kelvin. Hell, mass is a better example; there's a standard kilo in Paris which is shedding atoms, changing mass and causing potential issues for the metric system. Physical quantities are all the result of measurement on some conventional scale.
ReplyDeleteBut, if they were just after different unit systems, why wouldn't they choose an example of length measured in cm versus lenght measured in inches?
No idea. Maybe to exaggerate the difference between two measurements of the same object with two scales? Turvey never goes for the obvious example, it's a feature of his writing unfortunately.
The issue they are tackling is, what is the basis of measurement for the perception/action system? It's not going to be the units of physics, and thus any discrepancies between measurements made with these two systems is apples and oranges.
"Temperature is entirely a measurement on a scale. There are at least 3 in common use: Celsius, Fahrenheit and Kelvin."
ReplyDeleteBut, don't you agree that temperature was a physical quantity that existed before we came up with the Celsius, Fahrenheit, and Kelvin scales? So, that temperature is a real physical quantity, but temperatures scales are human creations?
But, also doesn't the choice of examples invoke what a psychologist might loosely call a confound? One wants examples that pin down the single feature that is in play, right? If you vary too much, you get a kind of confound.
ReplyDeleteBut, let me ask yet another question, do you have any references that expand upon this stuff about "chord geometry"? The TSRM is maddeningly obscure about this stuff.
ReplyDeleteThe point about measurement scales is that we have no privileged access to the 'real' physical thing temperature; we only have access to it via measurement devices which entail conventional scales. This fact is only a problem if you forget it, though.
ReplyDeleteI don't know anything about chord geometry in particular; but it looks like they're using it as an example of the fact that the same object (and the same properties of that object) can be described using different geometrical systems, which produce different results. It's another way of saying the same object might be 30cm or 12in long.
There's a lot of data around about how perceptual spaces, reach spaces, etc are often best described using non-Euclidean geometries (eg affine). If you use Euclid you would end up thinking people were making errors, but if you allow other geometries in you can see the basis for the behaviour (physicists do this sort of thing all the time). James Todd, Farley Norman, have done stuff one this; Geoff Bingham has some too.
Thanks for the clues, Andrew. I'll try to track some of this down.
ReplyDeleteI suppose that cognitivists are split on the idea of there being privileged access to anything, but I doubt many think that there is privileged access to temperature. (Bear in mind that TSRM are attributing assumptions to cognitivists/ inferentialists; they are not describing their own views.)
ReplyDeleteBut, it sounds as though you now agree that there is a difference between physical quantities, such as mass and temperature, and measurements of physical quantities, such as kilograms and degrees. Instead, you only want to deny privileged access. That's fine.
So long as you remember that perception is an act of measurement and that there is no reason to assume that that measurement is based in the scales physicists use.
ReplyDeleteAn example paper by Bingham using affine geometry; he cites a bunch of other relevant papers.
"So long as you remember that perception is an act of measurement and that there is no reason to assume that that measurement is based in the scales physicists use."
ReplyDeleteSo, I'll have to win you over one step at a time ... =) But, we've already established that.
Thanks for the link. It, of course, occur to me that postulating a perceptual space is kind of like postulating a system in which one represents the world. But, that's just how I roll. =)