At the beginning of the Summa Theologiae Aquinas defines...

At the beginning of the Summa Theologiae Aquinas defines sacra doctrina as a science that takes the articles of faith, the revelation received from God, as its principles.[^12] Sacra doctrina is probative ( argumentativa ), working from these principles to demonstrate other things, both through the exercise of human reasoning and the use of authorities, namely Sacred Scripture, the doctors of the Church and those philosophers, who have come to knowledge of truth by natural reason.[^13] For Aquinas the shape of such reasoning and the nature of the authorities appealed to depends on what those involved have in common, what principles drawn from revelation or human authorities they can agree on and hence reason from.

Because Jews accept the Old Testament, discussion with them can draw on this part of revelation as well as human reason and philosophical authorities. In the case of Muslims, there is no shared revelation and so discussion is limited to reasoning and use of philosophical authorities alone.[^14] Thus, Aquinas develops a scheme in which engagement with non-Western philosophy has a place within theology as the science of sacra doctrina.

This clearly does not make Aquinas an advocate of a liberal theology or a pluralist theology avant la lettre . In his account Islamic thought is given the same status as Greek philosophy, part of natural human philosophy.[^15] His comments about Muhammad reflect the extreme negativity of his time.[^16] Moreover, in his discussion of the probative work of sacra doctrina the emphasis is on disputation and the refutation of the views of others.

However, Aquinas’ actual use of Muslim or Jewish philosophy testifies to the importance they did have for his own theology. Aquinas does not, in fact, just set out to refute what Muslim or Jewish thinkers have to say. He also agrees with many of the points he finds in their accounts. Muslim and Jewish philosophy helps to shape and clarify his Christian theology as it develops and matures.

Their commentaries and independent works themselves become authorities to which he refers and with which he reasons as he constructs a Christian theology.