## Re: Quantifying over functions in first-order logic

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• I don t see why one would ever want to quantify over call_function. I don t think that there is an equivalent to that in regular logic. I can quantify over
Message 1 of 9 , Jun 14 7:43 AM
I don't see why one would ever want to quantify over call_function. I
don't think that there is an equivalent to that in "regular" logic.

I can quantify over reflexive by adding another level of indirection.
reflexive(f) becomes call_function_2(reflexive, f). As long as all
functions have a clearly defined level, it should work. Furthermore,
using levels is, from what I've read, a common way of avoiding
Russell's paradox in SOL. Does the "SOL is strictly more expressive
than FOL" claim apply only to SOL without levels?

Cedric

--- In aima-talk@yahoogroups.com, "Peter Norvig" <peter@...> wrote:
>
> I agree that for most of what you would reasonably want to do, it
> wouldn't matter. But the point is that when you quantify over all
> functions/relations (represented as objects), you wouldn't be
> quantifying over "call_function" nor "reflexive".
>
> -Peter
>
> On 6/13/06, cedricstjl <cedricstjl@...> wrote:
> > Why does it matter? I'm quantifying over "objects that behave exactly
> > like FOL functions" instead of quantifying directly over FOL
functions.
> >
> > I could write a program, which takes as input a second-order logic
> > sentence, does inference in FOL with the conversion from my first
> > post, and translates back into second-order logic. Or are there SOL
> > sentences that wouldn't be translatable?
> >
> > Thank you for your time,
> >
> > Cedric
> >
> > --- In aima-talk@yahoogroups.com, "Peter Norvig" <peter@> wrote:
> > >
> > > You could do that, and that could be a way of expressing
reflexivity,
> > > but it wouldn't be quantifying over FOL functions -- it would be
> > > quantifying over objects that you happen to call "functions" but are
> > > regular objects as far as the logic is concerned.
> > >
> > > -Peter
> > >
> > > On 6/8/06, cedricstjl <cedricstjl@> wrote:
> > > > I have some difficulty seeing the difference in expressiveness
between
> > > > first-order logic and higher-order logics. Wikipedia (as well
as AIMA,
> > > > AFAICT) says that F-O logic cannot quantify over functions. But
> > > > couldn't it be achieved equivalently by reifying functions and
> > > > predicates? I.e.:
> > > >
> > > > x = fun(var)
> > > >
> > > > becomes
> > > >
> > > > x = call_function(fun, var)
> > > >
> > > > and then one can express reflexivity:
> > > >
> > > > for all fun, a, b: reflexive(fun) <=> call_function(fun, a, b) =
> > > > call_function(fun, b, a)
> > > >
> > > > Cedric
• Actually, I think that I don t even need levels. for all fun, a, b: call_function(reflexive, fun) call_function(fun, a, b) = call_function(fun, b, a) seems
Message 2 of 9 , Jun 14 9:47 AM
Actually, I think that I don't even need levels.

for all fun, a, b: call_function(reflexive, fun) <=>
call_function(fun, a, b) = call_function(fun, b, a)

seems to work fine to me.

Cedric

--- In aima-talk@yahoogroups.com, "cedricstjl" <cedricstjl@...> wrote:
>
> I don't see why one would ever want to quantify over call_function. I
> don't think that there is an equivalent to that in "regular" logic.
>
> I can quantify over reflexive by adding another level of indirection.
> reflexive(f) becomes call_function_2(reflexive, f). As long as all
> functions have a clearly defined level, it should work. Furthermore,
> using levels is, from what I've read, a common way of avoiding
> Russell's paradox in SOL. Does the "SOL is strictly more expressive
> than FOL" claim apply only to SOL without levels?
>
> Cedric
>
> --- In aima-talk@yahoogroups.com, "Peter Norvig" <peter@> wrote:
> >
> > I agree that for most of what you would reasonably want to do, it
> > wouldn't matter. But the point is that when you quantify over all
> > functions/relations (represented as objects), you wouldn't be
> > quantifying over "call_function" nor "reflexive".
> >
> > -Peter
> >
> > On 6/13/06, cedricstjl <cedricstjl@> wrote:
> > > Why does it matter? I'm quantifying over "objects that behave
exactly
> > > like FOL functions" instead of quantifying directly over FOL
> functions.
> > >
> > > I could write a program, which takes as input a second-order logic
> > > sentence, does inference in FOL with the conversion from my first
> > > post, and translates back into second-order logic. Or are there SOL
> > > sentences that wouldn't be translatable?
> > >
> > > Thank you for your time,
> > >
> > > Cedric
> > >
> > > --- In aima-talk@yahoogroups.com, "Peter Norvig" <peter@> wrote:
> > > >
> > > > You could do that, and that could be a way of expressing
> reflexivity,
> > > > but it wouldn't be quantifying over FOL functions -- it would be
> > > > quantifying over objects that you happen to call "functions"
but are
> > > > regular objects as far as the logic is concerned.
> > > >
> > > > -Peter
> > > >
> > > > On 6/8/06, cedricstjl <cedricstjl@> wrote:
> > > > > I have some difficulty seeing the difference in expressiveness
> between
> > > > > first-order logic and higher-order logics. Wikipedia (as well
> as AIMA,
> > > > > AFAICT) says that F-O logic cannot quantify over functions. But
> > > > > couldn't it be achieved equivalently by reifying functions and
> > > > > predicates? I.e.:
> > > > >
> > > > > x = fun(var)
> > > > >
> > > > > becomes
> > > > >
> > > > > x = call_function(fun, var)
> > > > >
> > > > > and then one can express reflexivity:
> > > > >
> > > > > for all fun, a, b: reflexive(fun) <=> call_function(fun, a, b) =
> > > > > call_function(fun, b, a)
> > > > >
> > > > > Cedric
>
• ... This is a useful technique. I ve also seen it called currifying in other papers, however it doesn t let you achieve the full power of second-order logic,
Message 3 of 9 , Jun 14 1:02 PM
> On 6/8/06, cedricstjl <cedricstjl@> wrote:
> couldn't it be achieved equivalently by reifying functions and
> predicates?

This is a useful technique. I've also seen it called currifying in
other papers, however it doesn't let you achieve the full power of
second-order logic, much less higher-order logic.

A basic first-order logic result is that finiteness is not
expressible. That is there is no sentence P such that M is a model of
P iff. M is finite. However, in second order logic one can express
finiteness. Specifically, if P is the statement that all injective
functions are surjective, then M is a model of P iff. M is finite.

The wikipedia article on second-order logic discusses some of the
differences between first-order and second-order logic at more length
than this email.

Joe
• Hmmm, I ve got a hard time translating your example, or those from the Wikipedia article into logical sentences (I have no idea how to express that a model is
Message 4 of 9 , Jun 15 8:53 AM
Hmmm, I've got a hard time translating your example, or those from the
Wikipedia article into logical sentences (I have no idea how to
express that a model is finite, for instance). I guess I should pick
up a book on the subject; my only background in logic is from AIMA and

Thanks for the info!

Cedric

--- In aima-talk@yahoogroups.com, Joe Hendrix <jhendrix@...> wrote:
>
> > On 6/8/06, cedricstjl <cedricstjl@> wrote:
> > couldn't it be achieved equivalently by reifying functions and
> > predicates?
>
> This is a useful technique. I've also seen it called currifying in
> other papers, however it doesn't let you achieve the full power of
> second-order logic, much less higher-order logic.
>
> A basic first-order logic result is that finiteness is not
> expressible. That is there is no sentence P such that M is a model of
> P iff. M is finite. However, in second order logic one can express
> finiteness. Specifically, if P is the statement that all injective
> functions are surjective, then M is a model of P iff. M is finite.
>
> The wikipedia article on second-order logic discusses some of the
> differences between first-order and second-order logic at more length
> than this email.
>
> Joe
>
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