Thus we have the rule: No one is perfect unless he has thoroughly observed all the 613 precepts. If this be so, who is he and where is he that has observed all the 613 precepts? For even the lord of the prophets, Moses our Rabbi — peace be on him! — had not observed them all; for there are four obstacles which hinder one from observing all: (1.) There is the case of complete prevention, such as the law of the priesthood, the precepts of which only priests can observe, and yet these precepts are included in the 613.
Besides, there are among the number precepts appertaining to the Levites which concern neither priests nor Israelites, and also others which are binding on Israelites with which priests and Levites have nothing whatever to do. (2.) Then there are impossible cases, as, for instance, when one cannot observe the precept which enforces circumcision, because he has not a son to circumcise. (3 and 4.) There are also conditional and exceptional cases, as in the case of precepts having reference to the Temple and to the land of Israel.