Galen relied greatly on the doctrine of "critical days," which were thought to be influenced to some extent by the moon. His studies of the pulse were very useful to him in diagnosis. No doubt, he was an expert diagnostician mainly owing to his long, varied, and costly medical education, and his great natural powers of judgment. He asserted that with the help of the Deity he had never been wrong, but even his most ardent admirers would not be wanting in enthusiasm if they amended "never" into "hardly ever."