Ed Porter wrote:
Richard,
You just keep digging yourself in deeper.
Look at the original email in which you said "This is not correct." The
only quoted text that precedes it is quoted from me. So why are you saying
"Jim's statement was a misunderstanding"?
Okay, looks like some confusion here: the structure of Jim's message
was such that I thought the relevant comment came from him. Turns out
he was just quoting you. That's fine (sorry Jim): it just means that
you made the misleading statement.
Furthermore, I think your criticisms of my statements are generally
unfounded.
My choice of the word "reasoning" was not "not correct", as you imply, since
the Wikipedia definition says "Forward chaining is one of the two main
methods of REASONING when using inference rules." (Emphasis added.)
That is fair enough. I think it is a matter of taste, to some extent,
but I will take the rap for going against the Wikipedia gospel.
My statement made it clear I was describing the forward direction as being
from the if clause to the then clause, which matches the Wikipedia
definition, so what is "not correct" about that.
I did not say that this part of the text was incorrect.
In addition, you said my statement that in the absence of a temporal
criteria "the notion of what is forward and backward chaining might be
somewhat arbitrary" was a "completely incorrect conclusion."
Offensively strong language, considering it is unfounded.
Or, if it should turn out that it was well-founded, it would have been
quite polite and matter-of-fact to say "completely incorrect"
It is unfounded because in the absence of a temporal distinction, many
if-then rules, particularly if they are probabilistic, can viewed in a two
way form, with a probabilistic inference going both ways. In this case it
becomes unclear which side is the "if" clause, and which the "then" clause,
and, thus, unclear which way is forward and which backward by the definition
contained in Wikipedia --- unless there is a temporal criteria. This issue
becomes even more problematic when dealing with patterns based on temporal
simultaneity, as in much of object recognition, in which even a temporal
distinction, does not distinguish between what should be consider the if
clause and what should be considered the then clause.
Here is an example of backward chaining:
Start with a question: Is it true that "Socrates is mortal"?
Start by looking for any knowledge that allows us to conclude that
anything is or is not mortal. We search the KB and come up with these
candidates:
"If x is a plant, then x is mortal"
"If x is a rock, then x is not mortal"
"If x is a robot, then x is not mortal"
"If x lives in a post-singularity era, then x is not mortal"
"If x is a slug, then x is mortal"
"If x is a japanese beetle, then x is mortal"
"If x is a side of beef, then x is mortal"
"If x is a screwdriver, then x is not mortal"
"If x is a god, then x is not mortal"
"If x is a living creature, then x is mortal"
"If x is a goat, then x is mortal"
"If x is a parrot in a Dead Parrot Sketch, then x is mortal"
Now, before we go on to look at the second stage of this backward
chaining example, could you perhaps explain to me how "the absence of a
temporal distinction" applies or does not apply to any of these? I do
not believe that it is possible to reverse any of these rules, temporal
distinctions or any other distinctions.... you cannot say "if x is
mortal, then x is a plant", nor "if x is not mortal, then x lives in a
post-singularity era", etc etc etc.
In the process of backward chaining, the next step is to see if the
antecedents of any of these might allow us to connect up with Socrates
in some way, so we start with the first one, "If x is a plant" and try
to find out if anything allows us to conclude that Socrates is or is not
a plant. A search of the KB turns up these statements:
"If x contains chlorophyll, then x is a plant"
"If x is a dandelion, then x is a plant"
...... and on and on and on.
A couple of years later, after going several levels deep in its search,
the system finally digs deep enough in its knowledge base to come up
with the following chain of inference:
"Socrates contains blood"
"If x contains blood, then x will bleed when pricked"
"If x bleeds when pricked, then x is a man"
"If x is a man, then x owns footwear"
"If x owns footwear, then x is a living creature"
"If x is a living creature, then x is mortal"
And now, FINALLY, the backward chaining mechanism will be able to
conclude that "Socrates is mortal"
Please, Ed, could you explain to me how this typical case of backward
chaining could be reversed so that it becomes just a variation on
forward chaining?
The two mechanisms simply have different properties. If you were to try
to prove "Socrates is Mortal" by forward chaining, what would you do?
Start from a random point in your KB and start proving facts in random
order? How would you use the reversibility of the rules, which you
claim to exist, to set up a forward chaining proof, instead of using
backward chaining?
If there is no practical way to substitute forward chaining for backward
chaining, in what sense would it be true that one was just a kind of
variation on the other?
Hey, if I am wrong, I would love to hear some more about it, because I
will have learned something. So far, I have just been quoting the
established understanding of the meaning of these terms and wondering
why I, personally, am taking so much heat for it.
Enough of arguing about arguing. You can have the last say if you want. I
want to spend what time I have to spend on this list conversing with people
who are more concerned about truth than trying to sound like they know more
than others, particularly when they don't.
Anyone who reads this thread will know who was being honest and reasonable
and who was not.
Ed Porter
If I had a penny for every time you have accused me of being wrong, when
later discussion showed that I was quite correct, I'd have enough money
to build an AGI tomorrow.
Richard Loosemore
P.S. You know, it is interesting for me to ask myself "Why do you
correct people on the AGI list when they say something that you think is
wrong? Why not just ignore it and let them be wrong?" I suppose the
main reason I do it is that if I stay quiet and just let the list
degenerate into a gang of people talking nonsense to one another, with
nobody stepping in to point out the more obvious errors, then this list
will have become completely worthless.
-------------------------------------------
agi
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