Why do we say "An eye for an eye, a tooth for a tooth"?

Meaning:

The punishment should match the crime

Background:

The earliest known example of this expression is as one of the 282 laws of the Babylonian King Hammurabi (c.1790 BC).  The Code of Hammurabi was inscribed on huge stone slabs, known as steles - one example of which was discovered in in Iran in 1901 (having been plundered from Babylon in the 12th century BC).  It is now housed in the Louvre museum in Paris. 

The Code of Hammurabi is the oldest example of an almost complete set of ancient laws, but it is by no means the first set of laws. For example, the small sample still in existence of The Code of Ur-Nammu predates Hammurabi by at least 300 years.

Scholars believe that the earliest human legal systems were almost universally based on the principle of the law of retaliation (lex talionis) - that is the law of equal and direct retribution. 

Most scholars believe Hammurabi died in 1750 BC, and that his code was written in about 1786 BC (some scholars speculate that the biblical Nimrod and Hammurabi are one and the same).  This means that that Hammurabi lived about the same time as Abraham, and about four hundred years before Moses.

Which of course brings us back to the reason that this expression is so well known today - although the law itself is likely to date back to well before Hammurabi, the expression is so familiar to us today because of its frequent use in the Torah (the first five books of the Old Testament) where references to 'an eye for an eye' can be found in Exodus, Deuteronomy and Leviticus.

Anyone who kills an animal shall make restitution for it, life for life. Anyone who maims another shall suffer the same injury in return: fracture for fracture, eye for eye, tooth for tooth; the injury inflicted is the injury to be suffered."  Leviticus 24:18-20

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