R. G. Collingwood's philosophy of history is explained and critically evaluated. Collingwood advances an objective idealist historiography, according to which it is necessary for the historian to enter vicariously into the thoughts of historically interesting decision makers, literally re-thinking them in order to understand their reasoning in historical context. A detailed exposition of Collingwood's theory is presented, identifying its central features as they developed from the early to later periods of his philosophy. Collingwood's remarkable inversion of the positivist unity of sciences model is particularly emphasized, with its reduction of all the natural sciences and traditional subdivisions of philosophy to history, along with the aesthetics of historical narrative in selecting, explaining, and retelling the thoughts of historically interesting decision makers. Collingwood's philosophy of history is defended against a charge of vicious circularity, but, in light of other theoretical difficulties, a modified revisionary quasi-Collingwoodian or Collingwood-inspired historiography is finally proposed.