Reason and rationality are difficult conceptions to pin down. Encyclopedias of philosophy tend not to have specific articles devoted to them. When we look at what specific philosophers mean by reason and rationality, it quickly becomes obvious that they mean many different things. Most of the time philosophers claim to follow reason and rational methods, but it often seems that “rational” is no more than a philosopher's assertion that his methods and conclusions are obviously correct.
Consider that in the Enlightenment, “reason” meant a substitution of individual thought for inherited religious authority; for the medieval European philosophers it was a supplement to revelation; and for the Utilitarians it was the practical ideal of the greatest happiness for the greatest number. Modern relativism denies that reason can reach ultimate truth, and Romanticism rejects it in favor of a prerational experience of the world. It is not difficult to identify comparable competing notions of rationality in Islamic civilization. Clearly we are not dealing with a single, unambiguous concept. Therefore, if we are going to talk about reason in Islamic civilization, we need to make clear exactly which form – or, more likely, forms – of reason we are talking about.
Western ideas about reason are not the standard against which Islamic reason should be judged – there is, in any case, no single Western conception of reason to use as a touchstone – but Western intellectual history is diverse and thus unequalled as a point of comparison.