 Many viewed their project as an attempt to uncover principles of a mental language common to all thinkers. Aristotle had said, similarly, that spoken sounds symbolize “affections of the soul.” From this perspective, one expects to find some differences between propositions and overt sentences. If ‘Every star twinkles’ expresses a proposition that contains a copula, then spoken languages mask certain aspects of logical structure.  William of Ockham  held that a mental language would have no need for Latin’s declensions, and that logicians could ignore such aspects of spoken language. The ancient Greeks were aware of sophisms like the following: that dog is a father, and that dog is yours; so that dog is your father. This bad inference cannot share its form with the superficially parallel but impeccable variant: that dog is a mutt, and that mutt is yours; so that dog is your mutt. (See Plato, Euthydemus 298 d-e.) So the superficial features of sentences are not infallible guides to the logical forms of propositions. Still, the divergence was held to be relatively minor. Spoken sentences have structure; they are composed, in systematic ways, of words. And the assumption was that spoken sentences reflect the major aspects of propositional form, including a subject-predicate division. So while there is a distinction between the study of valid inference and the study of sentences used in spoken language, the connection between logic and grammar was thought to run deep. This suggested that the logical form of a proposition just is the grammatical form of some (perhaps mental) sentence.