Abstract
Second-order logic has a number of attractive features, in particular the strong expressive resources it offers, and the possibility of articulating categorical mathematical theories (such as arithmetic and analysis). But it also has its costs. Five major charges have been launched against second-order logic: (1) It is not axiomatizable; as opposed to first-order logic, it is inherently incomplete. (2) It also has several semantics, and there is no criterion to choose between them (Putnam, J Symbol Logic 45:464–482, 1980). Therefore, it is not clear how this logic should be interpreted. (3) Second-order logic also has strong ontological commitments: (a) it is ontologically committed to classes (Resnik, J Phil 85:75–87, 1988), and (b) according to Quine (Philosophy of logic, Prentice-Hall: Englewood Cliffs, 1970), it is nothing more than “set theory in sheep’s clothing”. (4) It is also not better than its first-order counterpart, in the following sense: if first-order logic does not characterize adequately mathematical systems, given the existence of non-isomorphic first-order interpretations, second-order logic does not characterize them either, given the existence of different interpretations of second-order theories (Melia, Analysis 55:127–134, 1995). (5) Finally, as opposed to what is claimed by defenders of second-order logic [such as Shapiro (J Symbol Logic 50:714–742, 1985)], this logic does not solve the problem of referential access to mathematical objects (Azzouni, Metaphysical myths, mathematical practice: the logic and epistemology of the exact sciences, Cambridge University Press, Cambridge, 1994). In this paper, I argue that the second-order theorist can solve each of these difficulties. As a result, second-order logic provides the benefits of a rich framework without the associated costs.
Similar content being viewed by others
Notes
The second-order existential quantifier is defined in the usual way: ∃X φ =def. ¬∀X ¬φ, and ∃f φ =def. ¬∀f ¬φ.
Shapiro provides further examples of mathematical concepts that can only be adequately expressed in second-order languages (1991, pp. 97–109).
As a further argument for second-order logic—at least for those with nominalist inclinations!—note that several nominalist proposals have adopted versions of this logic as part of their strategies to reduce ontological commitment to mathematical objects. This is the case of Field’s fictionalism [at least as presented in Field (1980), see Field (1989) for a first-order version] and Hellman’s modal-structural interpretation (Hellman 1989).
Monadic first-order logic has predicates with only one place, while monadic second-order logic has one-place predicates and second-order quantifiers binding one-place predicate variables.
Here is Putnam’s argument (1976, pp. 125–126). Let us assume that T is an ideal theory. It is consistent, complete, empirically adequate, simple, plausible etc. The only property of T that is left open (for the sake of argument) is its truth. (T may well be false.) Now, let us assume, with Putnam, that the world can be broken into infinitely many pieces, and that T states that there are infinitely many things. Since T is consistent, by the completeness theorem of first-order logic, it has a model. And since T has an infinite model, by the Löwenheim-Skolem theorem, it has a model of every infinite cardinality. Let us now select a model M that has the same cardinality as the world, and let us establish a one-to-one mapping from individuals of M into pieces of the world. We can use this mapping to define relations over M’s domain directly in the world, and as a result, we can define a satisfaction relation, sat, which establishes a ‘correspondence’ between T’s language and sets of pieces of the world. Now, provided that we interpret ‘true’ as true(sat)—that is, as the truth predicate that is defined by the relation sat, just as ‘true’ is defined in terms of ‘satisfies’ in Tarski’s account—the ideal theory T comes out true: indeed, it is true of the world. But then T cannot be false!
Kaplan’s proof of this fact is presented in Boolos (1984, pp. 432–433).
Supposing that the domain of discourse consists of critics, and Axy means ‘x admires y’, the Geach-Kaplan sentence becomes: ∃X (∃x Xx ∧ ∀x ∀y ((Xx ∧ Axy) → x ≠ y ∧ Xy)).
Boolos’s idea is that the informal metalanguage in which we give the semantics for (monadic) second-order logic contains the plural quantifier ‘there are objects’, which is then used to interpret the second-order monadic quantifiers. [For further discussion, see Shapiro (1991, pp. 222–226), and Simons (1997)].
As opposed to classical logic, paraconsistent logic distinguishes the notions of inconsistency and triviality. A theory T is inconsistent if it has a theorem of the form (A ∧ ¬A), and T is trivial if all the formulas of the language are theorems of T. The threat posed by inconsistencies in a classical logic setting is the triviality they bring. In a paraconsistent framework, however, there are inconsistent but non-trivial theories (see da Costa 1974; da Costa et al. 2007).
I am making here a distinction between naive set theory and an intuitive notion of set on the grounds that the former is a mathematical theory, whereas the latter incorporates intuitive, pre-theoretic assumptions that we may have about sets, collections etc.
As Shapiro remarks: “the major shortcoming of ω-languages is that they assume or presuppose the natural numbers. Therefore, such a language cannot be used to show, illustrate, or characterize how the natural number structure is itself understood, grasped, or communicated” (Shapiro 1985, p. 733).
References
Azzouni J (1994) Metaphysical myths, mathematical practice: the logic and epistemology of the exact sciences. Cambridge University Press, Cambridge
Azzouni J (2004) Deflating existential consequence: a case for nominalism. Oxford University Press, New York
Barwise J (ed) (1977a) Handbook of mathematical logic. North-Holland, Amsterdam
Barwise J (1977b) An introduction to first-order logic. In: Barwise (ed) [1977], pp 5–46
Boolos G (1975) On second-order logic. J Phil 72:509–527 (Reprinted in Boolos [1998], pp. 37–53)
Boolos G (1984) To be is to be a value of a variable (or to be some values of some variables). J Phil 81:430–449 (Reprinted in Boolos [1998], pp 54–72)
Boolos G (1985) Nominalist platonism. Phil Rev 94:327–344 (Reprinted in Boolos [1998], pp 73–87)
Boolos G (1998) Logic, logic, and logic. Harvard University Press, Cambridge, Mass
Bueno O (2005) Dirac and the dispensability of mathematics. Stud History Phil Mod Phy 36:465–490
Crossley J, Dummett M (eds) (1965) Formal systems and recursive functions. North-Holland, Amsterdam
da Costa NCA (1974) On the theory of inconsistent formal systems. Notre Dame J Formal Logic 15:497–510
da Costa NCA (1986) On paraconsistent set theory. Logique et Analyse 115:361–371
da Costa NCA, Krause D, Bueno O (2007) Paraconsistent logics and paraconsistency. In: Jacquette (ed) [2007], pp 791–911
Field H (1980) Science without numbers: a defense of nominalism. Princeton University Press, Princeton, NJ
Field H (1989) Realism, mathematics and modality. Basil Blackwell, Oxford
Hallett M (1984) Cantorian set theory and limitation of size. Clarendon Press, Oxford
Hellman G (1989) Mathematics without numbers: towards a modal-structural interpretation. Clarendon Press, Oxford
Jacquette D (ed) (2007) Philosophy of logic. North-Holland, Amsterdam
Melia J (1995) The significance of non-standard models. Analysis 55:127–134
Montague R (1965) Set theory and higher-order logic. In: Crossley, Dummett (eds) [1965], pp 131–148
Putnam H (1976) Realism and reason. In: Putnam [1978], pp 123–140
Putnam H (1978) Meaning and the moral sciences. Routledge and Kegan Paul, London
Putnam H (1980) Models and reality. J Symbol Logic 45:464–482 (Reprinted in Putnam [1983], pp 1–25)
Putnam H (1983) Realism and reason. Cambridge University Press, Cambridge
Quine WVO (1970) Philosophy of logic. Prentice-Hall, Englewood Cliffs, NJ
Resnik M (1988) Second-order logic still wild. J Phil 85:75–87
Resnik M (1997) Mathematics as a science of patterns. Oxford University Press, Oxford
Shapiro S (1985) Second-order languages and mathematical practice. J Symbol Logic 50:714–742
Shapiro S (1990) Second-order Logic, foundations, and rules. J Phil 87:234–261
Shapiro S (1991) Foundations without foundationalism: a case for second-order logic. Clarendon Press, Oxford
Shapiro S (1997) Philosophy of mathematics: structure and ontology. Oxford University Press, New York
Simons P (1982) Plural reference and set theory. In: Smith (ed) [1982], pp 199–256
Simons P (1997) Higher-order quantification and ontological commitment. Dialectica 51:255–271
Smith B (ed) (1982) Parts and moments. Philosophia, Munich
Author information
Authors and Affiliations
Corresponding author
Additional information
My thanks go to Philip Catton, Steven French, Sarah Kattau, Bernard Linsky, Judy Pelham, Jaroslav Peregrin, Peter Simons, Göran Sundholm, and Peter Woodruff for helpful discussions.
Rights and permissions
About this article
Cite this article
Bueno, O. A Defense of Second-Order Logic. Axiomathes 20, 365–383 (2010). https://doi.org/10.1007/s10516-010-9101-4
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s10516-010-9101-4