The language of thought hypothesis (LOTH), sometimes known as thought ordered mental expression (TOME), is a view in linguistics, philosophy of mind and cognitive science, put forward by American philosopher Jerry Fodor. It describes the nature of thought as possessing "language-like" or compositional structure (sometimes known as mentalese). On this view, simple concepts combine in systematic ways (akin to the rules of grammar in language) to build thoughts. In its most basic form, the theory states that thought, like language, has syntax.

Using empirical evidence drawn from linguistics and cognitive science to describe mental representation from a philosophical vantage-point, the hypothesis states that thinking takes place in a language of thought (LOT): cognition and cognitive processes are only 'remotely plausible' when expressed as a system of representations that is "tokened" by a linguistic or semantic structure and operated upon by means of a combinatorial syntax. Linguistic tokens used in mental language describe elementary concepts which are operated upon by logical rules establishing causal connections to allow for complex thought. Syntax as well as semantics have a causal effect on the properties of this system of mental representations.

These mental representations are not present in the brain in the same way as symbols are present on paper; rather, the LOT is supposed to exist at the cognitive level, the level of thoughts and concepts. The LOTH has wide-ranging significance for a number of domains in cognitive science. It relies on a version of functionalist materialism, which holds that mental representations are actualized and modified by the individual holding the propositional attitude, and it challenges eliminative materialism and connectionism. It implies a strongly rationalist model of cognition in which many of the fundamentals of cognition are innate.

The hypothesis applies to thoughts that have propositional content, and is not meant to describe everything that goes on in the mind. It appeals to the representational theory of thought to explain what those tokens actually are and how they behave. There must be a mental representation that stands in some unique relationship with the subject of the representation and has specific content. Complex thoughts get their semantic content from the content of the basic thoughts and the relations that they hold to each other. Thoughts can only relate to each other in ways that do not violate the syntax of thought. The syntax by means of which these two sub-parts are combined can be expressed in first-order predicate calculus.

The thought "John is tall" is clearly composed of two sub-parts, the concept of John and the concept of tallness, combined in a manner that may be expressed in first-order predicate calculus as a predicate 'T' ("is tall") that holds of the entity 'j' (John). A fully articulated proposal for what a LOT would have to take into account greater complexities such as quantification and propositional attitudes (the various attitudes people can have towards statements; for example I might believe or see or merely suspect that John is tall).

The language of thought hypothesis has been both controversial and groundbreaking. Some philosophers reject the LOTH, arguing that our public language is our mental language—a person who speaks English thinks in English. But others contend that complex thought is present even in those who do not possess a public language (e.g. babies, aphasics, and even higher-order primates), and therefore some form of mentalese must be innate. ^{[citation needed]}

The notion that mental states are causally efficacious diverges from behaviorists like Gilbert Ryle, who held that there is no break between mental state and behavior (as cause and effect). Rather, Ryle proposed that people act in some way because they are in a disposition to act in that way, that these causal mental states are representational. An objection to this point comes from John Searle in the form of biological naturalism, a non-representational theory of mind that accepts the causal efficacy of mental states. Searle divides intentional states into low-level brain activity and high-level mental activity. The lower-level, nonrepresentational neurophysiological processes have causal power in intention and behavior rather than some higher-level mental representation.^{[citation needed]}

Tim Crane, in his book The Mechanical Mind, states that, while he agrees with Fodor, his reason is very different. A logical objection challenges LOTH’s explanation of how sentences in natural languages get their meaning. That is the view that “Snow is white” is TRUE if and only if P is TRUE in the LOT, where P means the same thing in LOT as “Snow is white” means in the natural language. Any symbol manipulation is in need of some way of deriving what those symbols mean. If the meaning of sentences is explained regarding sentences in the LOT, then the meaning of sentences in LOT must get their meaning from somewhere else. Thus there seems to be an infinite regress of sentences getting their meaning. Sentences in natural languages get their meaning from their users (speakers, writers). Therefore, sentences in mentalese must get their meaning from the way in which they are used by thinkers and so on ad infinitum. This regress is often called the homunculus regress.