 Maritain’s position here contrasts sharply with that of his colleague, Etienne Gilson, who was more favourable to Bergson and who wrote little on the philosophy of nature throughout his life, arguably because he believed that the underlying Aristotelianism of the Thomistic position had been rendered irrelevant by modern science (Nelson 2007). Even sympathetic critics, such as Gerald McCool, argue that Maritain’s account of the three different degrees of abstraction that underlie this view is a distortion of Aquinas’s own view (McCool 1989: 254). More ardent critics have objected to Maritain’s account of what he calls ‘positivism’, and some, such as George Boas, argued at the time that Maritain depends on a theory of the existence of natural classes that have essences—a philosophical theory which may not, in fact, be presupposed by most scientists (Boas 1952). (Maritain, of course, would reply that this is precisely his point; such essences are not the object of empiriological sciences.)