The distinction between the historical and the philosophical is not the distinction between cause and effect: Cullen based his classificatory system on the attribution of related causes; nor is the distinction between principle and consequences, since Sydenham thought he was engaged in historical research when studying 'the way in which nature produces and sustains the different forms of diseases';' nor even is it exactly the difference between the visible and the hidden or conjectural, for one sometimes has to track down a 'history' that is enclosed upon itself and develops invisibly, like hectic fever in certain phthisics: 'reefs caught under water'.'