Our Gemara on Amud Aleph discusses the interesting status of metal that comes in contact with a corpse. A corpse, already the highest level of impurity, transmits impurity through metal that is in contact with it, at the same level. Ordinarily, a corpse which contacts another item will transmit impurity but also step down one level, which means that the chain erodes and eventually ends at levels 3-5 depending on whether it is for the holiest sacrificial food, kohen portions (terumah), or regular food (Chulin). What is the reason for this intensity of transmission with metal and a corpse?

The verse (Bamidbar 19:16) gives us a hint when it describes it as: “…anyone who touches a corpse of one who was killed by sword laying in the field…”

We can assume that when the written Torah uses a specific case to describe a law that will be broader in the oral Torah, the purpose of the text is to shine meaning on the law. For example the Torah forbids cooking meat and milk but the actual text discusses cooking a kid in its mother’s milk. We are not going to get into explaining this right now, but it is a good assumption that there is some core reason for forbidding the cooking of meat and milk in general that stems from its epitomized example of the kid in the mother’s milk. So too here, while it is true that all metal transmit the impurity, the deeper reason why, comes from the particularly noxious and destructive aspects of the sword that killed the corpse.

Let us take a deeper look. Recanati (ibid) hints at something primeval and evil. The field in the verse is the same field that Kayin killed Hevel, and the sword is the sword of Esav.

What is this really about? In general, we can understand the impact of corpses on the human psyche as a despair and feeling of emptiness. The Hebrew word for corpse is challal, which means empty (of soul). Without connecting to God this despair can overtake us, perhaps feeling lost in the impermanence of everything physical and spiral into nihilism. The more severe the loss, the longer and more severe the impurity. The impurities that require seven days seem to be a need to reprocess and go through a re-CREATION represented by the seven days of creation. Childbirth and Menstruation are also losses of life, life growing within and potential life, also require a recentering and re-creation and thus have similar impurity rituals. (This is an amalgam of ideas on how to understand ritual impurity, and is based on themes that have been developed by others, see for example, Rav Hirsch Bamidbar 19:13)

Keeping that in mind, we can see then why a corpse that was murdered by sword, and by symbolic extension, metal that comes in contact with a corpse, manifest the highest form of impurity. 

You might ask, still, I could understand an actual corpse that’s in contact with a sword, but why generalize it so much that it is metal so connected to a sword?

We see a similar phenomenon by the stones of the altar. The halachic requirement is not to use any metal instrument to fashion or hew the stones of the altar, but the traditional reason is not to have anything to do with warfare and death, which metal utensils often are used for. The verse itself seems to reference both the problem of using metal and its association with instruments of death (Shemos 20:22):

“And if you make for Me an altar of stones, do not build it of hewn stones; for by wielding your sword upon them you have profaned them.”

To understand this better, I think it’s important to tune ourselves toward the Torah’s ambivalence regarding technology. On the one hand, there is technology used in the Torah. Mathematical science, in order to calculate the movement of the planets and the moon, and artisanal expertise that was used in crafting the Mishkan and its utensils. On the other hand, the verses in Bereishis give credit to various tools manufactured by the descendants of Kayin, the first man who committed murder (see Bereishis 4:20-24.) Furthermore, although it’s not exactly stated in the text, what was the sin of the tower of Bavel? The simple reading indicates a difficulty with the hubris fostered by the technological achievement of building the tower.

Therefore, the extension of impurity from sword to all metal, and the similar application by the altar equating all metal to swords, might be hinting at the importance of recognizing the dangers of technology. Unfortunately, the military often drives technological innovation, or technological innovations are constantly used toward militaristic aims. Clearly technology can be used toward the advancement of material and even spiritual success by promoting health and welfare, but we must not let it turn into the Tower of Bavel.