It is in the latter sense that the Almighty is called “rock,” He being the origin and the causa efficiens of all things besides Himself. Thus we read, “He is the Rock, His work is perfect” (Deut. xxxii. 4); “Of the Rock that begat thee thou art unmindful” (Deut. xxxii. 18); “Their Rock had sold them” (xxxi. 30); “There is no rock like our God” (1 Sam. ii. 2): “The Rock of Eternity” (Isa. xxvi. 4). Again, “And thou shalt stand upon the Rock” (Exod. xxxiii. 21), i.e., Be firm and steadfast in the conviction that God is the source of all things, for this will lead you towards the knowledge of the Divine Being. We have shown (chap. viii.) that the words “Behold, a place is with me” (Exod. xxxiii. 21) contain the same idea. CHAPTER XVII Do not imagine that only Metaphysics should be taught with reserve to the common people and to the uninitiated; for the same is also the case with the greater part of Natural Science. In this sense we have repeatedly made use of the expression of the Sages, “Do not expound the chapter on the Creation in the presence of two” [vide Introd. page 2]. This principle was not peculiar to our Sages; ancient philosophers and scholars of other nations were likewise wont to treat of the principia rerum obscurely, and to use figurative language in discussing such subjects. Thus Plato and his predecessors called Substance the female, and Form the male. (You are aware that the principia of all existing transient things are three, viz., Substance, Form, and Absence of a particular form; the last-named principle is always inherent in the substance, for otherwise the substance would be incapable of receiving a new form; and it is from this point of view that absence [of a particular form] is included among the principia. As soon, then, as a substance has received a certain form, the privation of that form, namely, of that which has just been received, has ceased, and is replaced by the privation of another form, and so on with all possible forms, as is explained in treatises on natural philosophy.)—Now, if those philosophers who have nothing to fear from a lucid explanation of these metaphysical subjects still were in the habit of discussing them in figures and metaphors, how much more should we, having the interest of religion at heart, refrain from elucidating to the mass any subject that is beyond their comprehension, or that might be taken in a sense directly opposite to the one intended. This also deserves attention. CHAPTER XVIII