“In philosophy of mind, the language of thought hypothesis (LOTH) (...) our thoughts have a language-like structure that is independent of natural language. (...) There is a language of thought, and that it has a more logical form than ordinary natural language. This view has an added bonus: it tells us that, if you want to express yourself more clearly and more effectively in natural language, then you should express yourself in a form that is closer to computational logic - and therefore closer to the language of thought. (...)
Thoughts as represented in a “language” (sometimes known as mentalese) that allows complex thoughts to be built up by combining simpler thoughts in various ways. In its most basic form the theory states that thought follows the same rules as language: thought has syntax. (...) LOTH implies that the mind has some tacit knowledge of the logical rules of inference and the linguistic rules of syntax (sentence structure) and semantics (concept or word meaning). (...)
"Whether sensory or perceptual processes are to be treated within the framework of full-blown LOTH is again an open empirical question. It might be that the answer to this question is affirmative. If so, there may be more than one LOT realized in different subsystems or mechanisms in the mind/brain. (...) Propositional thought and thinking cannot be successfully accounted for in its entirety in purely imagistic terms. It claims that a combinatorial sentential syntax is necessary for propositional attitudes and a purely imagistic medium is not adequate for capturing that."
Via Andrea Graziano