But those also who have made considerable progress in the survey of the main principles ought to bear in mind the scheme of the whole system set forth in its essentials. For we have frequent need of the general view, but not so often of the detailed exposition. Indeed it is necessary to go back on the main principles, and constantly to fix in one’s memory enough to give one the most essential comprehension of the truth. And in fact the accurate knowledge of details will be fully discovered, if the general principles in the various departments are thoroughly grasped and borne in mind; for even in the case of one fully initiated the most essential feature in all accurate knowledge is the capacity to make a rapid use of observation and mental apprehension, and this can be done if everything is summed up in elementary principles and formulae. For it is not possible for anyone to abbreviate the complete course through the whole system, if he cannot embrace in his own mind by means of short formulae all that might be set out with accuracy in detail.