A language can be thought to contain four levels, corresponding to four features of human languages: sign, structure, semantics and pragmatics. While an alien language might not share all these features, it’s helpful to know what they are before we venture into extraterrestrial strangeness.

The language of an alien species who can communicate telepathically, for example, will not have the first level of signs. Aliens who have the cognitive capacity to remember an infinite number of signs (eg, a name) – each standing for a distinct meaning – would have no use for the second level of structure. Human languages, by contrast, have a structure because, despite our limited memory and cognitive abilities, it helps us create infinitely many sentences using finite elements.

Alien modes of communication may also have additional levels, ones that we cannot yet foresee. Perhaps there is an affective level that can encode how exactly one feels – say, the nature and intensity of one’s pain. Or a phenomenal level that can encode qualitative experiences, such as an apple’s redness.

An extraterrestrial language that lacks the third, semantic level would be particularly alien: one whose elements are not ‘about’ anything. Its words do not refer to objects nor are its sentences true or false descriptions of the world. Creatures that use such a language would be causal mechanisms that hook up with the world by way of environmental inputs – eg, smell, temperature or radiation – to produce resultant outputs. ‘Communication’ between such creatures may be a series of causal transactions: a stimulus from one causing a response in another, much like how hormones work in our bodies.

Even machines can employ signs, such as the high-pitched sounds of Gibberink, a language that some artificial intelligences use to interact.

Human languages, by contrast, have a structure because, despite our limited memory and cognitive abilities, it helps us create infinitely many sentences using finite elements.