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Common law is what has evolved as judges decide cases, one by one, over hundreds of years.
Unlike statutes, the common law is not written down in one place, but instead is deduced from the judges' writings. Under English common law, dogs were not considered to have any intrinsic value. They were kept, in the eyes of the law, merely for pleasure. This reasoning seems especially odd when you look at how many dogs were kept to catch rats, herd sheep or guard houses, but that's the way it was. Nowadays, the law in most places and for most purposes treats dogs just like other kinds of property.