A great many philosophical works have been lost; our limited knowledge of such lost works comes chiefly through the doxographical works of later philosophers, commentators, and biographers. Philosophers such as Plato and Aristotle also act as doxographers, as their comments on the ideas of their predecessors indirectly tell us what their predecessors' beliefs were. Plato's Defense of Socrates, for example, tells us much of what we know about the natural philosophy of Anaxagoras.
Islamic doxography is an aggregate of theosophical works (like Kitab al-Maqalat by Abu Mansur Al Maturidi) concerning the aberrations in Islamic sects and streams.