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same; even when surprise was not formalized, the informal explanation followed the same structure. The dynamic operator, “[]”, means that all the elements in the sequence up to i cannot have the property expressed by ϕ. This dynamic operator should be interpreted as a public announcement which eliminates all elements of the sequence up to i. For example, if the elements of the sequence are days of the week or hours in a day, then their order in the sequence is the chronological one. Therefore []¬◻ϕmeans that after all the days of the week or all the hours in a day up to theith have lost the possibility of having the feature that ϕ now attributes to i, the agent will not believe that i has that feature. This way of expressing the passing of days, hours, etc. through a dynamic operator represents one of the contributions DEL has made to the literature on the surprise examination paradox. Other authors simply index the regular belief or knowledge operators. For instance, Chow 1998 (15) writes Ka: “on the eve of the first day the students know ...”, see p. 47.

In this section I will not be concerned with what ϕ stands for, or with what exactly is the sequence that I mentioned in the last paragraph, but only with ways of altering the structure of the sentence expressing surprise and of varying the definition of the ◻. By “the structure of the sentence expressing surprise” I mean the string of logical connectives and operators that appear in that sentence. In the case of the standard way of defining surprise, the structure of the sentence is: proposition, and, dynamic operator, negation, a modal operator, proposition (same as before). It seems obvious, I would say, that the conjunction has to be where it is, after all surprise cannot emerge simply from our belief in something or from something being the case. It needs some sort of clash to come about, and the conjunction intuitively connects the two things that clash, thus generating the surprise. Furthermore, it also seems intuitive that we have the same proposition on both sides of the conjunction, with a negation in front of it on one side; after all, for the clash to occur there has to be a tension of some sort and this is between ϕand its negation. This restricts the ways in which the sentence can vary (for example, we cannot eliminate the negation or replace the conjunction with a disjunction, say). Therefore, I will explore some of the remaining different ways in which the structure of the surprise sentence can vary. I will argue against some types of variations and finally conclude that there isat least

one way of construing surprise which is not only intuitive but also makes it possible to come up with a solution to the surprise examination paradox that respects the constraints on a solution to this paradox discussed in the introduction.

It is important to remember that the purpose of this analysis of the different ways of defining surprise is to show how surprise could be formalized so that we get a solution that meets all of our pre-theoretical intuitions. Therefore, the purpose in this section is not to exhaust all the possibilities and defend only one, but rather the purpose is to familiarize the reader with the space of possibilities. I will, then, only

investigate a few ways of construing surprise. However, if one is not pleased with the definition I will finally argue for, and wishes to argue for another one, as long as that new definition also meets the pre-theoretical intuitions, I have nothing against it. The rationale is that, as I already mentioned in the introduction, the most impor- tant conditions to be met by a solution to the surprise examination paradox are the strong conditions and not the lax ones; formalizing surprise is a lax condition: it is dependent on our intuitions, and these can differ. So, as long as another interpreta- tion of surprise (which has some intuitive support) leads to a solution that meets all of the strong conditions, it represents, in my view, an equally good interpretation of surprise. To wit, we are not after the solution to the surprise examination paradox, but after one solution.

The common structure of surprise is taken to be: ϕ∧ []¬ ◻ϕ. Here are a few ways in which we can change this definition. I divide them in three categories. The first type of variation focuses on the second conjunct. The second type of variation focuses on the first conjunct, while the third type focuses on the exact interpretation of the ◻ operator.

The second conjunct could look in any of the following 6 ways: []¬ ◻ϕ, ¬[] ◻ϕ, ¬ ◻ []ϕ, [] ◻ ¬ϕ,◻[]¬ϕ, ◻¬[]ϕ.

The first conjunct could look in any of the following 5 ways: ϕ, ◻ϕ, []ϕ, [] ◻ϕ, ◻[]ϕ.

The meaning of the ◻ operator can be any of the following 5: (i) if defined on a Kripke frame: S5, KD45, K451_{; (ii) if defined on a plausibility frame: maximal, safe}

belief. Of course, this is again just part of the story, as one can very well define the operator syntactically; this is especially interesting if the point would be to analyze real-life situations in which the agents are not ideal (see Lorini and Castelfranchi 2007: the T est operatorbelow).

In between the options I have presented above (which again are just a small part of a larger number of available options, and we’ll see below one more type of variation) we have 510 structurally different ways of construing the surprise sentence. Each form of the second conjunct is combined with every form of the first conjunct and every definition of the◻operator. However, when both conjuncts contain a◻, it is by no means necessary that both operators are of the same type. That is: six variations of the first conjunct times three variations of the first conjunct that contain a◻times 25 (the number of distinct pairs formed from two types of ◻ operators, out of the five we consider here). This means 450 variations. To this we add six variations of the second conjunct times two variations of the first conjunct (those not containing a ◻ operator) times five different ◻ operators. This means 60 variations. In total, there are 510 different syntactic variations. Of course, some of these ways are not

2. THE NOTION OF “SURPRISE” 51