Purpose: What does mentalese refer to as a theory introduced in the science of language is the main concern of this paper. Method: The study is mainly descriptive where previous and related studies are reviewed and presented to reach a view about mentalese as an introduced theory in the science of language. Results: It has been argued for a long time whether we think in language or do we use language to think. Having known two or more languages, do you we think separately in each language? Does each language possesses a different place in our brains? Conclusions: There seems to be an agreement about the availability of mentalese as a linguistic faculty but there are different views about the nature and the interpretation of this faculty in relation to thought.