## Abstract

In the study of modal and nonclassical logics, translations have frequently been employed as a way of measuring the inferential capabilities of a logic. It is sometimes claimed that two logics are “notational variants” if they are translationally equivalent. However, we will show that this cannot be quite right, since first-order logic and propositional logic are translationally equivalent. Others have claimed that for two logics to be notational variants, they must at least be compositionally intertranslatable. The definition of compositionality these accounts use, however, is too strong, as the standard translation from modal logic to first-order logic is not compositional in this sense. In light of this, we will explore a weaker version of this notion that we will call *schematicity* and show that there is no schematic translation either from first-order logic to propositional logic or from intuitionistic logic to classical logic.

### Keywords

- Translation
- Notational variant
- Lindenbaum-Tarski algebras
- Compositionality
- Schematicity

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This thereby answers a question posed by Epstein [5, p. 388] in the affirmative. It is also straightforward to generate artificial counterexamples using any two partial orders such that neither is order-embeddable in the other.

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This follows from Theorems 23 and 24 below. There are also more direct proofs of these claims. For instance, suppose there were a compositional . Then where \(\varTheta \) is the \({\mathbf{{CPL}}}\)-schema such that , we have . Hence, for any \(\phi \in \mathcal {L}_\text {pred}\). But then for any \(\phi \in \mathcal {L}_\text {pred}\), .

- 8.
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Mossakowski et al. [11, p. 4] make this observation as well, though they do not offer any alternative notion in its place.

- 10.
The theorem cannot be extended to all normal modal logics, since there is a compositional translation from \(\mathbf S5 \) to \({\mathbf{{FOL}}}\) (setting ). It is unknown whether the result extends to other logics like \(\mathbf S4 \) that validate .

- 11.
The definition is inspired by the definition of “recursive” translations from [6, p. 16], who attributes the definition to Steven Kuhn.

- 12.
We could also require schematic translations to translate atomic formulas schematically. Such a constraint seems well-motivated, but it was not included in this definition for purposes of generality, as it was not necessary in the results to follow.

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## A Proof of Theorem 21

### A Proof of Theorem 21

Let \(\varTheta (\xi )\) be a first-order schema such that . Without loss of generality, we may assume \(\varTheta (\xi )\) is in (roughly) prenex normal form, i.e., that:

where \(\lambda \) and \(\mu \) are boolean combinations of atomic \({\mathbf{{FOL}}}\)-formulas and each . Observe that:

So .

First, we show . Using the fact that :

since \(y_1,\dots ,y_n\) are already bound in . So:

Hence, , and thus, .

Next, we show . Observe that:

So:

Thus, in particular, . Now, note that . Hence, unpacking :

Since , and since (given the last equivalence above), that means that for any \(\phi \) and \(\psi \). In particular, . Hence, . Thus, we have that . \(\square \)

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Kocurek, A.W. (2017). On the Concept of a Notational Variant. In: Baltag, A., Seligman, J., Yamada, T. (eds) Logic, Rationality, and Interaction. LORI 2017. Lecture Notes in Computer Science(), vol 10455. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-55665-8_20

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