The essay, with its analytical approach to the definition of political concepts, re-introduced the study of political philosophy to the methods of analytic philosophy. It is also one of Berlin's first expressions of his ethical ontology of value-pluralism. Berlin defined negative liberty (as the term "liberty" was used by Thomas Hobbes) as the absence of coercion or interference with agents' possible private actions, by an exterior social-body. He also defined it as a comparatively recent political ideal, which re-emerged in the late 17th century, after its slow and inarticulate birth in the Ancient doctrines of Antiphon the Sophist, the Cyrenaic discipleship, and of Otanes after the death of pseudo-Smerdis. In an introduction to the essay, Berlin writes: