R. Akiva says: This is one of the three things which Moses could not visualize until they were pointed out to him by the L–rd. Likewise, (Leviticus 11:29) "And this shall be unclean for you from among all the things that swarm upon the earth." (Likewise,) (Numbers 8:4) "And this is the work of the menorah." Others say: He was likewise perplexed as to slaughtering, viz. (Numbers 29:38) "And this is what you shall do upon the altar."