To put it at its most basic, I would summarise the historicist tradition in two propositions: 1) that history can be a science, and 2) that everything in the human world changes and is the product of history. We are all historicists today because we broadly accept both propositions. 1) would have been denied before the 18th century and Chladenius; and 2) would have been denied by many thinkers in the Enlightenment, who believed in a constant human nature, and by even more thinkers in the Middle Ages, who thought in terms of the eternal teachings of Christianity.

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I feel as though I ought to know something about this sort of stuff!