Theory developed by Jerry A Fodor, though going back to the English philosopher William of Ockham (c. 1285-1349).
It seeks to explain thinking by postulating a hypothetical language of thought (or mentalese) such that to have a belief or desire and so on is to be related in certain ways to one or more sentences of this language.
There are difficulties in spelling out these relations, and in saying how the items in mentalese - whatever form they may take - relate to the outer world which is being thought about.
We must not rely on analogies with ordinary languages, since these presuppose thinking while the language of thought is supposed to explain it. Its reliance on discrete items ('words') to correspond to the various bits of our thinking contrasts it with connectionism.
J A Fodor, The Language of Thought (1975)