For theologians and philosophers who died after about 900
the primary source is their own works, and these are now
relatively easy of access. Many of the most important works
are now in printed editions, and these are continually being
added to. There are also much wider facilities for obtaining
photographic reproductions of manuscripts. Carl Brockelmann's
Geschichte der arabischen Literatur (see Biblio-
graphy) aimed at providing a complete list of manuscripts
and printed editions; but of course it has nothing after its
date of publication ( 1943, 1949). It is in process of being
supplemented and brought up to date by Fuat Sezgin
Geschichte des arabischen Schrifttums, but that is progress-
ing only slowly. Details of printed books and of important
articles, sometimes with brief descriptions, are contained in
the Abstracta which constitute the second half of each
annual volume of the Revue des études islamiques.

For the earlier period only a few complete works exist, and
these mostly short, though further discoveries are made
from time to time. Much reliance has thus to be placed on
the secondary information derived from historians and other
writers, and notably from the heresiographers (writers of
accounts of the sects). The secondary sources have to be
handled cautiously and critically, especially since the names
of the sects were originally nicknames and could be used
differently by different people. It has also to be realized that
the material in the best-known works of heresiography
comes from Mu'tazilite and Ash'arite sources, and that in
other strands of Islamic theology many points were viewed
differently. In my book The Formative Period of Islamic
Thought I attempted a radical critique of the sources for the
early period, and I will here assume that this is accepted. I
will also omit detailed references to matters dealt with in
the Formative Period. The German translation of this work
has some small additions which take account of material
published after the English version went to print, notably
some works of Professor Josef van Ess of Tübingen dealing
with the Murji'ites and the Qadarites. The same volume also
contains a section on 'Islamische Theologie, 950-1850',
which is parallel to the second half of the present book.

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