A philosophic treatment of the history of philosophy takes an interest, like the merely learned, in the finest differences of systems, admits, with the sceptics, that they conflict with one another, and concedes to the eclectics that there is truth in all. Hence it neither loses sight of the thread of growing knowledge, like the first, nor regards the result as nil, like the second, nor, like the third, recognises in every system only pieces of developed truth, but the whole truth only in an undeveloped form. And thus it does not, like the first, beguile us into regarding philosophic doctrines as mere fancies and opinions, nor does it, like the second, shake the confidence in reason necessary to philosophy, nor lastly does it, like the eclectic method, make us indifferent towards dependence on a principle, i.e. towards systematic form.
Johann Eduard Erdmann, A History of Philosophy, Volume 1. Hough, tr. Macmillan (New York: 1893) pp. 3-4. Previously, Erdmann characterized the 'merely learned' approached as one that, practically speaking, treats all philosophical systems as equally true (thus treating the subject as a history of mere opinions), the skeptical approach as one that treats them all as equally false (thus treating the subject as a history of mere error), and the eclectic approach as one that treats all as delivering fragments of truth. In none of these three is the study of the history of philosophy treated as a properly philosophical enterprise in its own right.