Although the mitzva of sheḥita is a divine commandment whose full reason we cannot understand, its laws clearly express mercy. As R. Dr. Levinger explains in his comprehensive work Ha-Sheḥita Ve-tza’ar Ba’alei Ḥayim, sheḥita is the least painful way to put an animal to death without suffering. The artery that supplies most of the blood to the brain passes through the site of sheḥita, so immediately after sheḥita, the brain’s blood supply is cut off. By the time the pain signal would reach the cerebral cortex — where pain is consciously registered — the lack of blood and oxygen has already cut off the cortex’s ability to receive it. Even before the main artery is cut, the pain is barely felt: sheḥita must be performed with a sharp, smooth knife in a swift motion, and under these conditions the sensation of pain is almost imperceptible. In addition, the practice is to place the animal on its side before sheḥita. This reduces its distress, both because the position distracts the animal from the act of slaughter and because, in that position, the supply of blood and oxygen to the brain is cut off more quickly. Research has also shown that animals do not sense they are about to be killed: even after animals standing beside them were slaughtered, they continued calmly chewing grass — whereas when frightened, animals stop chewing entirely.
The requirements of sheḥita also appear to reflect the animal’s degree of development and capacity for suffering. The Torah is more demanding with domesticated and wild animals than with birds: for domesticated and wild animals, both simanim – the windpipe and the esophagus – must be cut, whereas for a bird, the sheḥita is valid, at the minimum level, if even one siman is cut (Ḥullin 27b). This distinction may be understood as follows: since birds have a less developed nervous system and their suffering is less acute, the law of their sheḥita is less demanding as well (above, 15:1, principle 2). The same logic may explain why fish and locusts require no sheḥita at all: since their nervous systems and brains are far less developed, the Torah did not command that they be slaughtered, and the prohibition of ever min ha-ḥai does not apply to them (below, section 11; above, 17:7–8).
A story is told about R. Yehuda Ha-Nasi (Rebbi), the editor of the Mishna. A calf was once being led to slaughter. It broke away from the shoḥet, hid its head in the folds of Rebbi’s garment, and cried out in distress. Although animals do not seem to sense that they are about to be killed, this calf apparently had been pulled forcefully and hurt, and it ran away in distress. Rebbi pushed the calf away and said, “Go to the shoḥet, for this is what you were created for.” At that moment, a severe accusation arose against Rebbi in Heaven: Since he did not have mercy on that calf, severe suffering should come upon him. For thirteen years, Rebbi suffered from gum pain and painful urination.
Sometime later, Rebbi’s maidservant was cleaning the house when she found a litter of young rats and was about to throw them out. Rebbi said to her: “Leave them, for it is stated, ‘His mercy is upon all His works’” (Tehilim 145:9). At that moment, Heaven judged him differently: Since he had shown mercy to animals, he too deserved mercy. His suffering ceased (Bava Metzia 85a).
Although halakha permits slaughtering animals for their meat, this story teaches that one should feel some measure of sorrow over it. In truth, in the world’s ideal state, human beings would have been able to sustain themselves on plant-based food alone. Only after the entire world descended from its original level through Adam’s sin and the sins that brought the Flood did the laws of nature take hold, and human beings began eating animals. Given Rebbi’s great spiritual and moral stature, he should have shown the calf some measure of mercy, soothing it and easing its path to slaughter. Because he was unmoved by its suffering, suffering came upon him (Rav Kook, Ḥazon Ha-tzimḥonut Ve-ha-shalom 1).
The Sages said that Rebbi did not resent his suffering. In his prayer, he asked that if it was proper for him to be refined through suffering, the suffering should continue to afflict him. Since this suffering came upon him in keeping with his moral stature, to refine and purify him, the Sages said that during all those years of his suffering, the rains did not cease (Bava Metzia 85a).
