The question of animal sacrifices during Temple times is something that can be difficult for us to understand today. According to Ramban (Nachmanides) the primary purpose of the sacrifice was that by being involved in the slaughter of an animal, the sinner bringing it would experience death vicariously. When the Kohen (priest) slaughtered the animal and burned it on the Altar, the aim was that the person bringing it would feel as though he himself had been killed and burned for having gone against the word of God.
 
God gave man the power of intellect so that he would be able to perfect himself. When a person sins, it is as if he has rejected his God-given intellect. Since the main thing distinguishing man from animals is his intellect, when a person sins, he is actually identifying with the animals. For this reason, an animal must be sacrificed.
 
On a Kabbalistic level, man consists of two elements, the animal, and the Divine; these two elements are in constant conflict with each other. While the Divine in man pulls him toward the spiritual, the animal in him draws him toward the physical and the mundane. When a person sins, he must therefore bring an animal as a sacrifice. By being an offering to God, the animal itself is elevated. At the same time, the animal in man, which can identify with this animal being sacrificed, is also elevated. The animal in man, which caused him to sin, is then brought back under the subjugation of the Divine.
 
All of these reasons merely touch the surface of the concept of sacrifices, which involves some of the deepest ideas of Judaism. It is obvious that the entire sacrificial system would appear brutal and barbaric unless administered in an almost perfect religious atmosphere. Only a nation of the highest moral and spiritual caliber could be worthy of it. That is why as a result of the moral laxity and spiritual degeneration of the Jewish people, the sacrificial system was eventually abolished.
 
Sacrifices could be offered only in one place, the Holy Temple in Jerusalem. This is explicitly prescribed in the Torah: "God shall choose a place for His Name to dwell; there shall you bring your offerings and sacrifices, your tithes and gifts" (Deuteronomy 12:11).
 
It is considered a most serious sin to offer a sacrifice outside the Jerusalem Temple. Here again, the reason is that it must be done in a place of the utmost holiness, so that the sacrificial system will not degenerate into something barbaric or brutal. The author of the Sefer HaChinuch (13th century) writes that killing an animal wantonly, if not done for food or in the worship of God in the proper place, is almost an act of murder. In this manner, the sacrificial system actually taught respect for all life, including that of animals.
 
All sacrifices had to be placed on the Altar, of which the Torah states, "You shall not build it of cut stones" (Exodus 20:22). The stones for the altar had to be dug up from deep in the ground, or taken from the depths of the sea, where they could not possibly have been in contact with human tools.
 
The reason for this is that cutting and shaping of stones is a human activity, based on man's intellectual ability, as distinguished from his animal nature. The altar was meant to rectify man's animal nature, and therefore had to be built entirely of natural stones.
 
Furthermore, the introduction of man's intellectual nature into the formation of these stones might sully them with his animal nature, as well. If the Altar itself was defiled by man's animal nature, it would not be able to rectify this element in man. In forbidding cut stones, the Torah therefore says, "For your sword has been lifted against it to desecrate it" (ibid.).
 

The sword represents the subjugation of man's intellect to his animal nature, because in war man makes use of his intellect to satisfy his animal instincts. This is the precise opposite of the function of the Altar, which was meant to subjugate the animal in man to his Divine element. If man's animal nature were allowed to interject, the entire sacrificial system represented by the Altar would become brutal and barbaric. Therefore, the Altar had to be made of stones as created by God, and not as modified by man.