Authorship analysis is the process of determining who produced a questioned text by language analysis. Although there has been significant success in the performance of computational methods to solve this problem in recent years, these are often methods that are not amenable to interpretation. Authorship analysis is in all effects an area of computer science with very little linguistics or cognitive science. This Element introduces a Theory of Linguistic Individuality that, starting from basic notions of cognitive linguistics, establishes a formal framework for the mathematical modelling of language processing that is then applied to three computational experiments, including using the likelihood ratio framework. The results propose new avenues of research and a change of perspective in the way authorship analysis is currently carried out.