 However, occasionalism was already an old doctrine at the time that Thomas Aquinas (1225–1274) wrote against it. (There is a helpful survey, in German, of the earlier history of occasionalism in Perler and Rudolph 2000.) Thomas indicated that the primary concern of the occasionalists was to strengthen the assertion of God’s omnipotence. Though he allowed that God must “concur” with creatures in producing effects, Thomas also claimed that there is reason to conclude that creatures are true secondary causes. For instance, he urged that it is more in accord with divine greatness to say that God communicates His power to creatures. Moreover, he claimed that it is simply evident to the senses that creatures have the power to bring about effects. Thomas also argued that if there were no natures in creatures that explain effects, there could be no true scientific explanation of effects through their natural causes.