Theories are not partially ordered

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This paper presents a simple example of first-order theories T1 and T2 such that i) T1 can be embedded in T2 and vice versa, ii) T1 posits all of the structure of T2 and vice versa, but iii) T1 and T2 are not equivalent. This shows that theories lack both the Cantor-Bernstein and co-Cantor-Bernstein properties and are neither partially ordered by the relation 'is embeddable in' nor by 'posits all of the structure of'. In addition, these results clarify the overall geography of notions of equivalence between theories and yield two philosophical payoffs related to the recent discussions of structure and equivalence.
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Archival date: 2020-01-15
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