The first of the four books opens with the trite contention that the art itself is not to be rejected because frequently abused by imposters, and with the admission that even the skilful investigator often makes mistakes owing to the incompleteness of human knowledge. In the first place, our doctrine of the nature of matter rests, Ptolemy says, more on conjecture than on certain knowledge. Secondly, old configurations of the stars cannot be safely used as the basis of present-day predictions. Indeed, so many are the different possible positions of the stars and the different possible arrangements of terrestrial matter in relation to the stars that it is difficult to collect enough instances on which to base judgment. Moreover, such things as diversity of place, of education and of custom must be reckoned with in foretelling the future of persons born under the same stars. But although predictions frequently fail, yet the art is not to be condemned any more than one rejects the art of navigation because of frequent shipwrecks. Thus far one might take Ptolemy for a well-balanced and accurate scientist in the modern sense of the term, but he does not maintain this level. After showing that it is useful to know the future and that astrology does not depend on fatal necessity, he proceeds to explain why the stars give knowledge of the future. This he intends to show from natural causes: _ubique naturalium causarum rationem sequentes_. This sounds well but his reasoning is superficial and childish, as his discussion of the influence exercised by the planets will indicate.