Mar 222014
 

Guy, not recognizably affiliated with the department, on the phone:

For a political philosopher to be in love with anyone is a sad thing.

Wonder what the other end of the conversation was like.

Feb 222014
 
guzzista

2013 Moto Guzzi V7 Stone.

 

For those who were wondering why there hadn’t been any new posts in a while: this beauty from the shores of Lake Como, that’s why.

m4s0n501
 Posted by at 16:22  Tagged with:
Oct 282013
 

The paper on the general interpretation of first-order quantifiers has now been published online by the Review of Symbolic Logic (this direct link requires subscriber/institutional access, or email me for a PDF). The philosophical companion to that paper — “Life on the Range” — is not out yet, but a pre-print can be found here.

 Posted by at 10:04
Aug 232013
 

Alfred Tarski’s great technical innovation in his Wahrheitsbegriff (1933/36) was the introduction of the auxiliary notion of satisfaction of a first-order formula \varphi(x_1,\ldots,x_n) by a function s assigning objects from some model M‘s domain  to the variables \{x_i : i \ge 0\}  of the language. Once the notion of satisfaction is in place, one can then define truth of a sentence \varphi in a model M, by saying that \varphi is true just in case it is satisfied by some (equivalently: every) assignment. It is this latter notion of truth that is of real mathematical and philosophical interest, of course, but it cannot be defined directly by recursion because in a first-order language (contrary to the propositional case), sentences are not built up from other sentences by means of constructors such as connectives. Rather, it is formulas in general that are so built, with sentences providing a special case. The characteristic clause of satisfaction is the one for quantified formulas:

  • A formula \forall y \, \varphi(x_1, \ldots, x_n,y) is satisfied by an assignment s in M if and only if the formula \varphi(y, x_1, \ldots, x_n) is satisfied, in M, by every assignment that that differs from s at most in the value s(y).

The detour through satisfaction, while technically interesting, it is just that — a detour. It can also sometimes get pedagogically problematic, as students usually grasp the importance of truth while not always appreciating the necessity of satisfaction.

There is an alternative approach, which does away with the notion of satisfaction, but uses a variant notion of a model. According to this approach, we only allow models that are “rich” in that every object a \in M is the denotation of a constant c_a in the language. The truth clause for the universal quantifier is then as follows:

  • A sentence \forall y \, \varphi(y) is true in M if and only if \varphi(c_a) is true in M for each constant c_a.

(I believe this definition is fairly standard in some textbooks, but I can’t track down a definition now, any references appreciated). While this definition sidesteps the notion of satisfaction, there are some drawbacks:

  1. The definition only applies to “rich” models, while models in which some elements are not denoted by constants are also very natural. Moreover, we are forced to consider truth of sentence \varphi in a model having a different signature than \varphi itself.
  2. The language changes with the model: for each model M, the definition applies only to the expanded language \mathcal{L}_M introducing new constants for each member of M.
  3. The definition forces us to consider uncountable languages, in order to assess truth in uncountable models.
  4. The expanded signature affects the available automorphism of a model. For example, the model (\mathbb{Z}, <) has infinitely many automorphisms in the signature with the relation < only, but only one if we add the constant 0.

So here is a way of combining these two approaches in such a way that:

  1. There is no detour through satisfaction.
  2. The language is countable and independent of the model in which a sentence is evaluated for truth (although the language will be expanded, just not in a way determined by the model).

Fix a basic language \mathcal{L}, and let c_1,c_2,\ldots be a countable set of new individual constants. For each n\ge 0 let \mathcal{L}_n = \mathcal{L} \cup \{c_1,\ldots,c_n\}  (so that \mathcal{L}_0 = \mathcal{L}). We define the notion of truth by induction on \mathcal{L}_n-sentences, simultaneously for every n. The quantifier case:

  • An \mathcal{L}_n-sentence \forall y \, \varphi(c_1,\ldots,c_n,y) is true in the \mathcal{L}_n-structure M if and only if the \mathcal{L}_{n+1}-sentence \varphi(c_1,\ldots,c_n,c_{n+1}) is true in every \mathcal{L}_{n+1}-expansion of M (i.e., if  \varphi(c_1,\ldots,c_n,c_{n+1}) is true in every \mathcal{L}_{n+1}-structure M' that differs from M only in that it assigns a denotation to c_{n+1}).

So, when \varphi is an \mathcal{L}-sentence, the definition gives us a direct account of the truth or falsity of \varphi is a model having the same signature.

 Posted by at 12:30
Aug 042013
 

On July 24 the system-wide senate of the University of California formally adopted an Open Access Policy (PDF). Under the policy, each faculty member across the ten campuses

grants the University of California a nonexclusive, irrevocable, worldwide license to exercise any and all rights under copyright relating to each of his or her scholarly articles, in any medium, and to authorize others to do the same, for the purpose of making their articles widely and freely available in an open access repository.

Under the policy, faculty will upload a “final version” of their scholarly work to eScholarship, a public repository hosted by the California Digital Library. “Final version” means the post-peer-review version of the manuscript before the publisher’s typesetting and finalizing (although some publishers allow the typeset version to be posted). Faculty have the option to opt out of the policy altogether (for each instance) or delay public access to their work. The University provides an “Addendum” that faculty can use when returning copyright transfer to the publishers in order to alert publishers that the work is subject to a pre-existing license (the license applies by default, even if the Addendum is not returned to the publisher, unless the faculty member explicitly opts out of the policy).

The policy will go into effect on Nov. 1 for UCLA, UCI and UCSF, and a year later at all remaining campuses. More information can be found on the Open Access website and in the Frequently Asked Questions.

Needless to say, this is good all-around.

 Posted by at 10:13
Apr 302013
 

There is an old paradox, originally due to Montague (JSL 20 (2), 1955, p.140) that only requires the most minimalist set theoretic apparatus, viz., the existence of singletons. It does not seem to be very well known, so here it is.

Let A = \{ x | \forall k (x \in k \to \exists y \in k(k \cap y = \varnothing))\}. Intuitively, A is the collection of all sets that only belong to well-founded sets. Clearly, A is too big to be a set, but that is not the point. The point is that this can be shown in classical predicate logic using only existence of singletons:

    \[ \forall x \exists y \forall z(z \in y \leftrightarrow  z = x).\]

We proceed by cases, and derive a contradiction in each.

  1. A \in A; then A only belongs to well-founded sets. Since \{A\} exists and A \in \{A\}, the latter must be well-founded, so \exists y \in \{A \}(\{A\} \cap y = \varnothing). But y \in \{A\} is equivalent to y=A, so \{A\} \cap A = \varnothing, which is impossible if A \in A (for then A itself is in the intersection \{A\} \cap A).
  2. A \notin A. Then there must be a k^* such that: A \in k^* and \forall y \in k^*(k^* \cap y \not= \varnothing). In particular k^* \cap A \not= \varnothing, so there is a z which belongs to both k^* and A. Now, since z\in A:

        \[\forall k (z \in k \to \exists y \in k(k \cap y = \varnothing)),\]

    and since z\in k^*, we have \exists y \in k^*(k^* \cap y = \varnothing) contradicting the choice of k^*.

 Posted by at 10:34