"Knowledge is immense and life is short: therefore it is not obligatory to learn all the sciences, such as Astronomy and Medicine, and Arithmetic, etc., but only so much of each as bears upon the religious law: enough astronomy to know the times (of prayer) in the night, enough medicine to abstain from what is injurious, enough arithmetic to understand the division of inheritances and to calculate the duration of the Iddat. (see Idda)]"