 Gregory’s view relies on the traditional medieval notion (going back to Aristotle’s On Interpretation 1, 16a3–5) of “mental language”, the “language of thought” that underlies and is expressed in spoken and written  language.[30]  Unlike spoken and written languages, where the signification of words and propositions is thoroughly a matter of convention, signification in mental language is fixed by nature once and for all, the same for everyone. Consequently, propositions in mental language can never signify otherwise than they “normally” do. It follows that an analysis like Bradwardine’s or Heytesbury’s, or the modified Heytesbury solution, according to which insolubles do signify otherwise than they normally do, cannot be applied to propositions formed in mental language. Although neither Bradwardine nor Heytesbury seems to have drawn this conclusion, it follows from their theories that insolubles cannot be formulated in mental language.