Vacuous truths in Superstructure approach to Nonstandard Analysis












2












$begingroup$


Good evening everybody,



at the moment I'm studying non-standard analysis, specifically the superstructure approach to it. This approximately works as described in chapter 3 of http://people.dm.unipi.it/dinasso/papers/20.pdf. So far I've mostly understood this construction, and also worked through the detailed construction in chapter 4.4 of Chang & Keisler's Model Theory. But there is one unmentioned detail that has been bugging me, concerning the interpretation of formulas in $V(X)$, the superstructure over a base set $X$.



One of the central points of nonstandard analysis is the transfer principle, which states that for a bounded quantifier formula $varphi (x_1,...,x_n)$, if $a_1,...,a_n in V(X)$, then $$(V(X), in) models varphi[a_1,...,a_n] iff (V(^* ! X), in) models varphi[^*a_1,...,{} ^*a_n].$$



Since we only interpret these formulas in $V(X)$, this means that when considering a bounded quantifier, say the quantifier in $forall w in u (w in v)$ (or, phrased differently, $u subseteq v$), the quantifier ranges only over elements in $u cap V(X)$. In most cases, this will not be a problem, since $V(X)$ is transitive, so if $u in V(X) setminus X$, then $u cap V(X) = u$.



But if we consider any element $x$ of the base set $X$ (so $x cap V(X) = emptyset$) and an arbitrary $y in V(X)$, then $(V(X), in)$ 'believes' $forall w in x (w in y)$ vacuously, although this usually isn't true in our normal universe. I think this is suboptimal, since the goal should be proving valid formulas (in our usual set theoretic setting) by proving them inside $V(X)$ and $V(^* ! X)$, using the transfer principle and other tools.



I first tried the naive fix of just allowing assignments $a_1,...,a_n in V(X) setminus X$, but this does not handle nested quantifications, for example in the statement $forall x in X(forall z in x(z in y))$, where $y in V(X)$ is arbitrary and $X$ is our base set. Also, apart from when they are the range of a quantifier, we need to be able to assign individuals (relative to $V(X)$) to free variables.



It seems like we need to avoid any quantification over elements of rank $0$ to be able to infer truth of $varphi$ (in our set theoretic universe) from truth of $varphi$ in $(V(X), in)$. So, my questions are as follows:



$1.$ First, and foremost: Am I missing something? Is this an actual problem or just a misunderstanding?



$2.$ If not, why wasn't this mentioned in the texts I considered? Also, is there a simpler characterization of the formulas that will be 'misinterpreted' by $(V(X), in)$?



Thank you for your time.










share|cite|improve this question











$endgroup$

















    2












    $begingroup$


    Good evening everybody,



    at the moment I'm studying non-standard analysis, specifically the superstructure approach to it. This approximately works as described in chapter 3 of http://people.dm.unipi.it/dinasso/papers/20.pdf. So far I've mostly understood this construction, and also worked through the detailed construction in chapter 4.4 of Chang & Keisler's Model Theory. But there is one unmentioned detail that has been bugging me, concerning the interpretation of formulas in $V(X)$, the superstructure over a base set $X$.



    One of the central points of nonstandard analysis is the transfer principle, which states that for a bounded quantifier formula $varphi (x_1,...,x_n)$, if $a_1,...,a_n in V(X)$, then $$(V(X), in) models varphi[a_1,...,a_n] iff (V(^* ! X), in) models varphi[^*a_1,...,{} ^*a_n].$$



    Since we only interpret these formulas in $V(X)$, this means that when considering a bounded quantifier, say the quantifier in $forall w in u (w in v)$ (or, phrased differently, $u subseteq v$), the quantifier ranges only over elements in $u cap V(X)$. In most cases, this will not be a problem, since $V(X)$ is transitive, so if $u in V(X) setminus X$, then $u cap V(X) = u$.



    But if we consider any element $x$ of the base set $X$ (so $x cap V(X) = emptyset$) and an arbitrary $y in V(X)$, then $(V(X), in)$ 'believes' $forall w in x (w in y)$ vacuously, although this usually isn't true in our normal universe. I think this is suboptimal, since the goal should be proving valid formulas (in our usual set theoretic setting) by proving them inside $V(X)$ and $V(^* ! X)$, using the transfer principle and other tools.



    I first tried the naive fix of just allowing assignments $a_1,...,a_n in V(X) setminus X$, but this does not handle nested quantifications, for example in the statement $forall x in X(forall z in x(z in y))$, where $y in V(X)$ is arbitrary and $X$ is our base set. Also, apart from when they are the range of a quantifier, we need to be able to assign individuals (relative to $V(X)$) to free variables.



    It seems like we need to avoid any quantification over elements of rank $0$ to be able to infer truth of $varphi$ (in our set theoretic universe) from truth of $varphi$ in $(V(X), in)$. So, my questions are as follows:



    $1.$ First, and foremost: Am I missing something? Is this an actual problem or just a misunderstanding?



    $2.$ If not, why wasn't this mentioned in the texts I considered? Also, is there a simpler characterization of the formulas that will be 'misinterpreted' by $(V(X), in)$?



    Thank you for your time.










    share|cite|improve this question











    $endgroup$















      2












      2








      2





      $begingroup$


      Good evening everybody,



      at the moment I'm studying non-standard analysis, specifically the superstructure approach to it. This approximately works as described in chapter 3 of http://people.dm.unipi.it/dinasso/papers/20.pdf. So far I've mostly understood this construction, and also worked through the detailed construction in chapter 4.4 of Chang & Keisler's Model Theory. But there is one unmentioned detail that has been bugging me, concerning the interpretation of formulas in $V(X)$, the superstructure over a base set $X$.



      One of the central points of nonstandard analysis is the transfer principle, which states that for a bounded quantifier formula $varphi (x_1,...,x_n)$, if $a_1,...,a_n in V(X)$, then $$(V(X), in) models varphi[a_1,...,a_n] iff (V(^* ! X), in) models varphi[^*a_1,...,{} ^*a_n].$$



      Since we only interpret these formulas in $V(X)$, this means that when considering a bounded quantifier, say the quantifier in $forall w in u (w in v)$ (or, phrased differently, $u subseteq v$), the quantifier ranges only over elements in $u cap V(X)$. In most cases, this will not be a problem, since $V(X)$ is transitive, so if $u in V(X) setminus X$, then $u cap V(X) = u$.



      But if we consider any element $x$ of the base set $X$ (so $x cap V(X) = emptyset$) and an arbitrary $y in V(X)$, then $(V(X), in)$ 'believes' $forall w in x (w in y)$ vacuously, although this usually isn't true in our normal universe. I think this is suboptimal, since the goal should be proving valid formulas (in our usual set theoretic setting) by proving them inside $V(X)$ and $V(^* ! X)$, using the transfer principle and other tools.



      I first tried the naive fix of just allowing assignments $a_1,...,a_n in V(X) setminus X$, but this does not handle nested quantifications, for example in the statement $forall x in X(forall z in x(z in y))$, where $y in V(X)$ is arbitrary and $X$ is our base set. Also, apart from when they are the range of a quantifier, we need to be able to assign individuals (relative to $V(X)$) to free variables.



      It seems like we need to avoid any quantification over elements of rank $0$ to be able to infer truth of $varphi$ (in our set theoretic universe) from truth of $varphi$ in $(V(X), in)$. So, my questions are as follows:



      $1.$ First, and foremost: Am I missing something? Is this an actual problem or just a misunderstanding?



      $2.$ If not, why wasn't this mentioned in the texts I considered? Also, is there a simpler characterization of the formulas that will be 'misinterpreted' by $(V(X), in)$?



      Thank you for your time.










      share|cite|improve this question











      $endgroup$




      Good evening everybody,



      at the moment I'm studying non-standard analysis, specifically the superstructure approach to it. This approximately works as described in chapter 3 of http://people.dm.unipi.it/dinasso/papers/20.pdf. So far I've mostly understood this construction, and also worked through the detailed construction in chapter 4.4 of Chang & Keisler's Model Theory. But there is one unmentioned detail that has been bugging me, concerning the interpretation of formulas in $V(X)$, the superstructure over a base set $X$.



      One of the central points of nonstandard analysis is the transfer principle, which states that for a bounded quantifier formula $varphi (x_1,...,x_n)$, if $a_1,...,a_n in V(X)$, then $$(V(X), in) models varphi[a_1,...,a_n] iff (V(^* ! X), in) models varphi[^*a_1,...,{} ^*a_n].$$



      Since we only interpret these formulas in $V(X)$, this means that when considering a bounded quantifier, say the quantifier in $forall w in u (w in v)$ (or, phrased differently, $u subseteq v$), the quantifier ranges only over elements in $u cap V(X)$. In most cases, this will not be a problem, since $V(X)$ is transitive, so if $u in V(X) setminus X$, then $u cap V(X) = u$.



      But if we consider any element $x$ of the base set $X$ (so $x cap V(X) = emptyset$) and an arbitrary $y in V(X)$, then $(V(X), in)$ 'believes' $forall w in x (w in y)$ vacuously, although this usually isn't true in our normal universe. I think this is suboptimal, since the goal should be proving valid formulas (in our usual set theoretic setting) by proving them inside $V(X)$ and $V(^* ! X)$, using the transfer principle and other tools.



      I first tried the naive fix of just allowing assignments $a_1,...,a_n in V(X) setminus X$, but this does not handle nested quantifications, for example in the statement $forall x in X(forall z in x(z in y))$, where $y in V(X)$ is arbitrary and $X$ is our base set. Also, apart from when they are the range of a quantifier, we need to be able to assign individuals (relative to $V(X)$) to free variables.



      It seems like we need to avoid any quantification over elements of rank $0$ to be able to infer truth of $varphi$ (in our set theoretic universe) from truth of $varphi$ in $(V(X), in)$. So, my questions are as follows:



      $1.$ First, and foremost: Am I missing something? Is this an actual problem or just a misunderstanding?



      $2.$ If not, why wasn't this mentioned in the texts I considered? Also, is there a simpler characterization of the formulas that will be 'misinterpreted' by $(V(X), in)$?



      Thank you for your time.







      logic set-theory model-theory nonstandard-analysis






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      edited Feb 3 at 1:48









      Hanul Jeon

      17.7k42881




      17.7k42881










      asked Jan 30 at 20:57









      user2103480user2103480

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          $begingroup$

          This has a lot to do with language. $V(X)$ is an $mathcal{L}$-structure of language $mathcal{L}={=,in,V(X)}$ where $"="$ and $"in"$ are predicates and $V(X)$ is a set of constants.



          For $V(X)$ to be an $mathcal{L}$-structure you need an interpretation of your language. So for every constant $xin V(X)$ we need to associate an element $yin V(X)$, clearly we can pick $y=x$. For our predicates $"in"$ and $"="$ we have to a find subsets $M_{in},M_{=}subset V(X)times V(X)$. We then say for $"ain b"$ or $"c=d"$ are true if and only if $(a,b)in M_{in}$ and $(c,d)in M_{=}$ respectively. Again you can pick the obvious subsets $M_{=}={(x,x)in V(X)times V(X)$ and $M_{in}={(x,y)in V(X)times V(X):xin y}$. These symbols along with logical symbols as $rightarrow,wedge,neg,vee,forall$, etc are the only symbols we may use to write statements.



          The statement $(forall xin X)(forall zin x)(zin y)$ where $y$ is some constant would be rewritten as
          $$(forall x)(xin Xrightarrow (forall z)(zin xrightarrow zin y))$$
          where $y$ and $X$ are constants.



          Now the question remains, is this formula true. This does depend on your interpretation of the $"in"$ predicate. But if you interpret it in such a way that $(x,y)notin M_{in}$ for all $yin X$, then this is a true statement. $zin x$ is not a true for $xin x$, so $(forall z)(zin xrightarrow zin y)$ is true by definition of $rightarrow$ This then makes $(forall x)(xin Xrightarrow (forall z)(zin xrightarrow zin y))$ true again due to the definition of $rightarrow$.
          Basically this statement is true in the same way that the statement: "every element of the empty set is a vampire" is true. You will never be able to avoid these kind of silly true formula's, they are just a consequence of writing a rigorous logical language, but they are true in both $V(X)$ and set theory.






          share|cite|improve this answer









          $endgroup$









          • 1




            $begingroup$
            I you are interested I actually wrote my master thesis on non-standard analysis and give a very basic introduction into language, $mathcal{L}$-structures and models and how they interact with superstructures: universiteitleiden.nl/binaries/content/assets/science/mi/…
            $endgroup$
            – Floris Claassens
            Feb 7 at 20:29












          Your Answer





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          1 Answer
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          active

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          active

          oldest

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          2





          +100







          $begingroup$

          This has a lot to do with language. $V(X)$ is an $mathcal{L}$-structure of language $mathcal{L}={=,in,V(X)}$ where $"="$ and $"in"$ are predicates and $V(X)$ is a set of constants.



          For $V(X)$ to be an $mathcal{L}$-structure you need an interpretation of your language. So for every constant $xin V(X)$ we need to associate an element $yin V(X)$, clearly we can pick $y=x$. For our predicates $"in"$ and $"="$ we have to a find subsets $M_{in},M_{=}subset V(X)times V(X)$. We then say for $"ain b"$ or $"c=d"$ are true if and only if $(a,b)in M_{in}$ and $(c,d)in M_{=}$ respectively. Again you can pick the obvious subsets $M_{=}={(x,x)in V(X)times V(X)$ and $M_{in}={(x,y)in V(X)times V(X):xin y}$. These symbols along with logical symbols as $rightarrow,wedge,neg,vee,forall$, etc are the only symbols we may use to write statements.



          The statement $(forall xin X)(forall zin x)(zin y)$ where $y$ is some constant would be rewritten as
          $$(forall x)(xin Xrightarrow (forall z)(zin xrightarrow zin y))$$
          where $y$ and $X$ are constants.



          Now the question remains, is this formula true. This does depend on your interpretation of the $"in"$ predicate. But if you interpret it in such a way that $(x,y)notin M_{in}$ for all $yin X$, then this is a true statement. $zin x$ is not a true for $xin x$, so $(forall z)(zin xrightarrow zin y)$ is true by definition of $rightarrow$ This then makes $(forall x)(xin Xrightarrow (forall z)(zin xrightarrow zin y))$ true again due to the definition of $rightarrow$.
          Basically this statement is true in the same way that the statement: "every element of the empty set is a vampire" is true. You will never be able to avoid these kind of silly true formula's, they are just a consequence of writing a rigorous logical language, but they are true in both $V(X)$ and set theory.






          share|cite|improve this answer









          $endgroup$









          • 1




            $begingroup$
            I you are interested I actually wrote my master thesis on non-standard analysis and give a very basic introduction into language, $mathcal{L}$-structures and models and how they interact with superstructures: universiteitleiden.nl/binaries/content/assets/science/mi/…
            $endgroup$
            – Floris Claassens
            Feb 7 at 20:29
















          2





          +100







          $begingroup$

          This has a lot to do with language. $V(X)$ is an $mathcal{L}$-structure of language $mathcal{L}={=,in,V(X)}$ where $"="$ and $"in"$ are predicates and $V(X)$ is a set of constants.



          For $V(X)$ to be an $mathcal{L}$-structure you need an interpretation of your language. So for every constant $xin V(X)$ we need to associate an element $yin V(X)$, clearly we can pick $y=x$. For our predicates $"in"$ and $"="$ we have to a find subsets $M_{in},M_{=}subset V(X)times V(X)$. We then say for $"ain b"$ or $"c=d"$ are true if and only if $(a,b)in M_{in}$ and $(c,d)in M_{=}$ respectively. Again you can pick the obvious subsets $M_{=}={(x,x)in V(X)times V(X)$ and $M_{in}={(x,y)in V(X)times V(X):xin y}$. These symbols along with logical symbols as $rightarrow,wedge,neg,vee,forall$, etc are the only symbols we may use to write statements.



          The statement $(forall xin X)(forall zin x)(zin y)$ where $y$ is some constant would be rewritten as
          $$(forall x)(xin Xrightarrow (forall z)(zin xrightarrow zin y))$$
          where $y$ and $X$ are constants.



          Now the question remains, is this formula true. This does depend on your interpretation of the $"in"$ predicate. But if you interpret it in such a way that $(x,y)notin M_{in}$ for all $yin X$, then this is a true statement. $zin x$ is not a true for $xin x$, so $(forall z)(zin xrightarrow zin y)$ is true by definition of $rightarrow$ This then makes $(forall x)(xin Xrightarrow (forall z)(zin xrightarrow zin y))$ true again due to the definition of $rightarrow$.
          Basically this statement is true in the same way that the statement: "every element of the empty set is a vampire" is true. You will never be able to avoid these kind of silly true formula's, they are just a consequence of writing a rigorous logical language, but they are true in both $V(X)$ and set theory.






          share|cite|improve this answer









          $endgroup$









          • 1




            $begingroup$
            I you are interested I actually wrote my master thesis on non-standard analysis and give a very basic introduction into language, $mathcal{L}$-structures and models and how they interact with superstructures: universiteitleiden.nl/binaries/content/assets/science/mi/…
            $endgroup$
            – Floris Claassens
            Feb 7 at 20:29














          2





          +100







          2





          +100



          2




          +100



          $begingroup$

          This has a lot to do with language. $V(X)$ is an $mathcal{L}$-structure of language $mathcal{L}={=,in,V(X)}$ where $"="$ and $"in"$ are predicates and $V(X)$ is a set of constants.



          For $V(X)$ to be an $mathcal{L}$-structure you need an interpretation of your language. So for every constant $xin V(X)$ we need to associate an element $yin V(X)$, clearly we can pick $y=x$. For our predicates $"in"$ and $"="$ we have to a find subsets $M_{in},M_{=}subset V(X)times V(X)$. We then say for $"ain b"$ or $"c=d"$ are true if and only if $(a,b)in M_{in}$ and $(c,d)in M_{=}$ respectively. Again you can pick the obvious subsets $M_{=}={(x,x)in V(X)times V(X)$ and $M_{in}={(x,y)in V(X)times V(X):xin y}$. These symbols along with logical symbols as $rightarrow,wedge,neg,vee,forall$, etc are the only symbols we may use to write statements.



          The statement $(forall xin X)(forall zin x)(zin y)$ where $y$ is some constant would be rewritten as
          $$(forall x)(xin Xrightarrow (forall z)(zin xrightarrow zin y))$$
          where $y$ and $X$ are constants.



          Now the question remains, is this formula true. This does depend on your interpretation of the $"in"$ predicate. But if you interpret it in such a way that $(x,y)notin M_{in}$ for all $yin X$, then this is a true statement. $zin x$ is not a true for $xin x$, so $(forall z)(zin xrightarrow zin y)$ is true by definition of $rightarrow$ This then makes $(forall x)(xin Xrightarrow (forall z)(zin xrightarrow zin y))$ true again due to the definition of $rightarrow$.
          Basically this statement is true in the same way that the statement: "every element of the empty set is a vampire" is true. You will never be able to avoid these kind of silly true formula's, they are just a consequence of writing a rigorous logical language, but they are true in both $V(X)$ and set theory.






          share|cite|improve this answer









          $endgroup$



          This has a lot to do with language. $V(X)$ is an $mathcal{L}$-structure of language $mathcal{L}={=,in,V(X)}$ where $"="$ and $"in"$ are predicates and $V(X)$ is a set of constants.



          For $V(X)$ to be an $mathcal{L}$-structure you need an interpretation of your language. So for every constant $xin V(X)$ we need to associate an element $yin V(X)$, clearly we can pick $y=x$. For our predicates $"in"$ and $"="$ we have to a find subsets $M_{in},M_{=}subset V(X)times V(X)$. We then say for $"ain b"$ or $"c=d"$ are true if and only if $(a,b)in M_{in}$ and $(c,d)in M_{=}$ respectively. Again you can pick the obvious subsets $M_{=}={(x,x)in V(X)times V(X)$ and $M_{in}={(x,y)in V(X)times V(X):xin y}$. These symbols along with logical symbols as $rightarrow,wedge,neg,vee,forall$, etc are the only symbols we may use to write statements.



          The statement $(forall xin X)(forall zin x)(zin y)$ where $y$ is some constant would be rewritten as
          $$(forall x)(xin Xrightarrow (forall z)(zin xrightarrow zin y))$$
          where $y$ and $X$ are constants.



          Now the question remains, is this formula true. This does depend on your interpretation of the $"in"$ predicate. But if you interpret it in such a way that $(x,y)notin M_{in}$ for all $yin X$, then this is a true statement. $zin x$ is not a true for $xin x$, so $(forall z)(zin xrightarrow zin y)$ is true by definition of $rightarrow$ This then makes $(forall x)(xin Xrightarrow (forall z)(zin xrightarrow zin y))$ true again due to the definition of $rightarrow$.
          Basically this statement is true in the same way that the statement: "every element of the empty set is a vampire" is true. You will never be able to avoid these kind of silly true formula's, they are just a consequence of writing a rigorous logical language, but they are true in both $V(X)$ and set theory.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Feb 7 at 20:26









          Floris ClaassensFloris Claassens

          1,33229




          1,33229








          • 1




            $begingroup$
            I you are interested I actually wrote my master thesis on non-standard analysis and give a very basic introduction into language, $mathcal{L}$-structures and models and how they interact with superstructures: universiteitleiden.nl/binaries/content/assets/science/mi/…
            $endgroup$
            – Floris Claassens
            Feb 7 at 20:29














          • 1




            $begingroup$
            I you are interested I actually wrote my master thesis on non-standard analysis and give a very basic introduction into language, $mathcal{L}$-structures and models and how they interact with superstructures: universiteitleiden.nl/binaries/content/assets/science/mi/…
            $endgroup$
            – Floris Claassens
            Feb 7 at 20:29








          1




          1




          $begingroup$
          I you are interested I actually wrote my master thesis on non-standard analysis and give a very basic introduction into language, $mathcal{L}$-structures and models and how they interact with superstructures: universiteitleiden.nl/binaries/content/assets/science/mi/…
          $endgroup$
          – Floris Claassens
          Feb 7 at 20:29




          $begingroup$
          I you are interested I actually wrote my master thesis on non-standard analysis and give a very basic introduction into language, $mathcal{L}$-structures and models and how they interact with superstructures: universiteitleiden.nl/binaries/content/assets/science/mi/…
          $endgroup$
          – Floris Claassens
          Feb 7 at 20:29


















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