Languages that allow a "vacuous" past tense morpheme in complement clauses of attitude verbs  are  referred to  as SOT (sequence-of-tense) languages, and languages that do not  are referred to as non-SOT languages. This squib observes that quite generally, if in a given language the present tense morpheme obligatorily refers to the utterance time  in  present-under-past sentences, that language allows a "vacuous" past. I show that in order to account for this correlation, any theory of SOT has to be supplemented with a principle of Universal Grammar that requires every well-formed matrix sentence to be "embeddable" under a propositional attitude verb. I call this principle the Embeddability Principle.