By empty theory I mean the theory In the empty language ( ie no non logical symbols ) with no axioms.
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The sentences of the empty theory over the empty language include only the logical connective, quantifiers and equality, so deciding them is quite simple. The empty theory over a language which includes only single-argument predicates is monadic logic, which is also decidable. The empty theory over a language which includes at least one predicate with two arguments is, however, not decidable.
The theory of linear orders should be decidable using quantifier elimination.