In the later nineteenth century the success of the natural sciences had spread the idea that one method should apply to all fields of knowledge; Comte's positivism proposed studying society as one studies physics. In his Introduction to the Human Sciences of 1883, Wilhelm Dilthey objected — not by retreating into irrationalism but by establishing a different criterion of rigour.

His distinction rests on two verbs: the natural sciences *erklären*, explain; the human sciences *verstehen*, understand. A falling stone is explained by causes; a legal text, a ritual, a poem or a revolution is understood by being traced back to the lived experience, intention and historical context of those who produced it. The reason this is possible is simple for Dilthey: what is studied was made by human beings too, so investigator and object share a ground of lived experience (Erlebnis). Understanding is a re-living carried out on that ground.

Dilthey also gave one of the most consistent statements of historicism: human nature is not a fixed essence but something formed in history, so the route to understanding people runs through history rather than psychology. The thesis is the systematic continuation of the road Vico opened a century and a half earlier. Its influence was wide: Weber's interpretive sociology, Husserl's life-world, Heidegger's analysis of existence and Gadamer's hermeneutics were all built on this distinction. Qualitative research methods, historiography and cultural studies still work with the question Dilthey asked: is measuring a human work the same as understanding it?