[From Bruce Abbott (960911.1705 EST)]
Rick Marken (960911.0900) --
Purpose n 1. something set up as an object or end to be attained 2. an
action in course of execution (Webster's New Collegiate Dictionary, 1980).
The HPCT reference signal seems to correspond exactly to the first dictionary
definition of "purpose"; the reference signal is a specification that is "set
up" (by another control system) as an object or end to be attained. By this
definition, the purpose of the thermostat is not a temperature (which is a
controlled varable); it's the reference specification (implemented as the
location of contacts) for the perceptual representation of the temperature
(size of the bimetallic strip).
Yes, the purpose is the end _to be attained_, not the end in and of itself.
The purpose of the control system is not the value of the reference signal
(e.g., 72 degrees F) but the end to be attained (i.e., bring the room
temperature up to 72 degrees F and hold it there).
So, according to you and Simon, it seems that an object has a "purpose" if it
is 1) well suited to producing a particular result and 2) this suitability
was selected. These seem like pretty subjective criteria to me. How do I test
to determine whether an object has purpose? Did the structures that we now
call wings have a purpose before they could function as wings? Do objects
have only one purpose at a time? Does the purpose of an object change
suddenly or gradually? Did the structures that were to become wings suddenly
have a new purpose as soon as they could be used to fly or did they gradually
gain this purpose, overlapping perhaps with their previous purpose (if they
had one), in evolutionary history? What is the scientific merit of your
opinion that the wing has only one real "purpose" -- flight?
I wish you wouldn't put words in my mouth. "What is the scientific merit of
your opinion that the wing has only one real 'purpose' -- flight," you ask.
Now this assumes that somewhere in my writing I have asserted that the wing
has only one real purpose, doesn't it? What I did say is that the _main_
purposes of the wing are to provide lift, thrust, and control (during
flight), and this is hardly the same thing as asserting that these are the
_only_ purposes for which wings have evolved.
For most artifacts created by human beings, we can ask the designer or other
knowledgable person what the artifact is "for" (and there may be multiple
answers); we discover the intention of its designer. We learn that
screwdrivers are for driving screws and, if we are properly taught, that
they should NOT be used for chizeling as this tends to damage the
screwdriver and render it unsuitable for its intended purpose. But the
screwdriver has characteristics that may suit it to other purposes (I might
use a big one as a substitute tent stake, for example).
If there are no knowledgable people around to provide a hint, I may have
great difficulty discovering the purpose an artifact was designed to serve.
I recently visited a museum in which several rather mysterious implements
were on display and the visitor was invited to guess that they were for. I
can tell you that I didn't have a clue. Still, if you told me what the
purpose of each implement is, I could probably easily identify which
features of the implement contribute to its ability to serve that purpose.
Discovering the purpose from the characteristics of the artifact is much
more difficult: a matter of inference. Archeologists often have great
difficulty determining what purpose some artifact was designed to serve.
But the fact is that the intended purpose of an artifact often can be
inferred from its structure and organization. The job is even easier if you
can see the object "at work," doing what it was designed to do.
Now when we turn our attention to "natural" artifacts, I grant that there is
no entity out there with any intentions, no designer and thus no purpose in
the dictionary sense of intended use. Yet the structures that evolve still
have that same character, AS IF they had been designed for a purpose. The
reason they are as they are is that, over the course of evolution, these
were the "solutions" that worked: that aided in the survival of the
organism. If the bird's purpose (end to be attained) is to fly, it is able
to do so only because its wings have been moulded over the course of
evolution so as to exquisitely serve this purpose. They are not so well
suited to other purposes for which the bird may employ them, such as beating
Objects whose characteristics have been shaped in this way can be described
in ways that objects not so shaped cannot be. We can ask of them, what does
this or that feature contribute to the function it serves, given the
environment in which it must operate? In the control system, what is the
function of the sensor, the comparator, the reference signal, the output
mechanism? What is the function of the system as a whole? In objects not
so shaped, their characteristics are merely their characteristics. They are
as they are, and that's it. In biological organisms the characteristics
have been so shaped. We can ask, what is skin for, and why is the skin of
the seal backed up by a thick layer of blubber, whereas the skin of a dog is
not? What is the end these features help these animals to attain, the
purpose of these features?
If you're going to insist that there is no scientific way to discover the
answer, then you'll have to explain how biologists have been able to figure
out those purposes for so many biological structures.