thinking is a form of inner speech. This view is sometimes termed psychological nominalism. It states that thinking involves silently evoking words and connecting them to form mental sentences. The knowledge a person has of their thoughts can be explained as a form of overhearing one's own silent monologue. Three central aspects are often ascribed to inner speech: it is in an important sense similar to hearing sounds, it involves the use of language and it constitutes a motor plan that could be used for actual speech. This connection to language is supported by the fact that thinking is often accompanied by muscle activity in the speech organs. This activity may facilitate thinking in certain cases but is not necessary for it in general. According to some accounts, thinking happens not in a regular language, like English or French, but has its own type of language with the corresponding symbols and syntax. This theory is known as the language of thought hypothesis. Inner speech theory has a strong initial plausibility since introspection suggests that indeed many thoughts are accompanied by inner speech. But its opponents usually contend that this is not true for all types of thinking. It has been argued, for example, that forms of daydreaming constitute non-linguistic thought. This issue is relevant to the question of whether animals have the capacity to think. If thinking is necessarily tied to language then this would suggest that there is an important gap between humans and animals since only humans have a sufficiently complex language. But the existence of non-linguistic thoughts suggests that this gap may not be that big and that some animals do indeed think. Language of thought hypothesis There are various theories about the relation between language and thought. One prominent version in contemporary philosophy is called the language of thought hypothesis. It states that thinking happens in the medium of a mental language. This language, often referred to as Mentalese, is similar to regular languages in various respects: it is composed of words that are connected to each other in syntactic ways to form sentences. This claim does not merely rest on an intuitive analogy between language and thought. Instead, it provides a clear definition of the features a representational system has to embody in order to have a linguistic structure. On the level of syntax, the representational system has to possess two types of representations: atomic