Guide for the Perplexed, by Moses Maimonides, Friedländer tr. [1904], at sacred-texts.com
THE word ẓur (rock) is a homonym. First, it denotes "rock," as "And thou shalt smite the rock" (ẓur) (Exod. xvii. 6). Then, "hard stone," like the flint, e.g., "Knives of stone" (ẓurim) (Josh. V. 2). It is next employed to signify the quarry from which the stones are hewn; comp. "Look unto the rock (ẓur) whence ye are hewn" (Isa. li. 1). From this latter meaning of the term another figurative notion was subsequently derived, viz., "the root and origin" of all things. It is on this account that after the words "Look to the rock whence ye are hewn," the Prophet continues, "Look unto Abraham your father," from which we evidently may infer that the words "Abraham your father" serve to explain "the rock whence ye are hewn"; and that the Prophet meant to say, "Walk in his ways, put faith in his instruction, and conduct yourselves according to the rule of his life! for the properties contained in the quarry should be found again in those things which are formed and hewn out of it."
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.