# understanding

**URL:** <http://discourse.iapct.org/t/understanding/15317>\
**Category:** CSG1998\
**Created:** [February 16, 1998, 6:40pm UTC](http://discourse.iapct.org/t/understanding/15317 "1998-02-16T18:40:21Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Martin\_Taylor4](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/m/71c47a/32.png) [@Martin\_Taylor4](http://discourse.iapct.org/u/Martin_Taylor4)\
**Post date:** [February 16, 1998, 6:40pm UTC](http://discourse.iapct.org/t/understanding/15317/1 "1998-02-16T18:40:21Z")

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[Martin Taylor 980216 13:10]

> Bill Powers (980215.0300 MST) to Martin Taylor 980214 14:20

> It looks as though you are sincerely determined not  
> to understand what I mean.

Gordon Pask described three levels of understanding, of which the second  
was the ability to paraphrase. Now, I \_think\_ I understand what you are  
trying to say, but to check it, let me provide a paraphrase of the part of  
your message that follows the disagreement about the historical root of  
the topic of psychophysics.

Point 1. Psychophysical experiments cannot tell you the nature of the  
perception being tested.

Point 2. The Test cannot tell you the nature of the perception being  
tested, for the same reasons that apply to the psychophysical experiments.

Point 3. The above invalidates any data obtained from psychphysical  
experiments, at least insofar as they might be claimed to say something  
about the subject, because not all the relevant variables might have  
been considered the first time.

Point 4. The above argues that the Test is a valid way to determine  
something useful about the subject, because it can be done again if  
later data suggest that not all the relevant variables had been considered  
the first time.

Have I got that right? I think I understand what you mean. And perhaps  
why you say it. But I don't understand the logic used to construct the  
argument.

Martin

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**Author:** ![Bill\_Powers1](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/b/8e7dd6/32.png) [@Bill\_Powers1](http://discourse.iapct.org/u/Bill_Powers1)\
**Post date:** [February 16, 1998, 9:42pm UTC](http://discourse.iapct.org/t/understanding/15317/2 "1998-02-16T21:42:12Z")

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[From Bill Powers (980216.1440 MST)]

Martin Taylor 980216 13:10--

> Now, I \_think\_ I understand what you are  
> trying to say, but to check it, let me provide a paraphrase of the part of  
> your message that follows the disagreement about the historical root of  
> the topic of psychophysics.
> 
> Point 1. Psychophysical experiments cannot tell you the nature of the  
> perception being tested.
> 
> Point 2. The Test cannot tell you the nature of the perception being  
> tested, for the same reasons that apply to the psychophysical experiments.

So far, so good. All that we can say about the perception is in the form of  
a model.

> Point 3. The above invalidates any data obtained from psychphysical  
> experiments, at least insofar as they might be claimed to say something  
> about the subject, because not all the relevant variables might have  
> been considered the first time.

This is not what I have said. Some kinds of psychophysical data have been  
obtained in circumstances where the subject was in control of the  
perception, so whether the experimenter had a good model of what was being  
perceived or not, the \_subject\_ certainly did, and we can be sure that the  
controlled perception was in the state the subject intended.

Some psychophysical experiments satisfy most of the conditions of the Test,  
and we can be reasonably sure that the results are valid. Those that did  
not satisfy those conditions need to be reviewed.

To repeat, I have never said that all psychophysical experiments are invalid.  
That is your interpretation. What I have said is that since we don't know  
whether a valid procedure was carried out, in most cases, we have to look  
at them all again before accepting any of them. Regardless of how many  
thousands of papers have been written.

4. The above argues that the Test is a valid way to determine

> something useful about the subject, because it can be done again if  
> later data suggest that not all the relevant variables had been considered  
> the first time.

That is not the only reason the Test is useful. The Test can reveal when  
the subject is NOT perceiving what you thought was being perceived, and  
that is something that open-loop experimenter manipulations can't do. If  
you apply a stimulus open-loop and you get a response, all you can assume  
is that the stimulus was perceived by the subject. There is no way to show  
that some other stimulus was in fact affected by the first one, so it was  
the disturbance of the second stimulus that actually led to the response.  
Nor, in the usual approach, is there any indication that the response (even  
yes/no) might be affecting something that is disturbed by the apparent  
stimulus and stabilized by the response.

This is only to be expected. An experimenter who knew nothing about control  
theory couldn't have been thinking in terms of controlled perceptions.  
Control theory brings a new set of terms into the equation, and this  
necessarily calls for some re-examination of old results.

I'm not sure why the idea of re-doing earlier experiments in the light of  
new theoretical considerations evokes such strenuous opposition. Are we so  
sure that these studies will stand up to a review that even to suggest  
looking at them again is insulting? What IS the problem, anyway?

Best,

Bill P.

> **···**
>
> > Have I got that right? I think I understand what you mean. And perhaps  
> > why you say it. But I don't understand the logic used to construct the  
> > argument.
> > 
> > Martin