The Mesopotamian tradition of extispicy of dead sacrificial sheep remained remarkably consistent throughout Old Babylonian, Middle Babylonian and Neo-Assyrian times. It was the main method of divination along with astrology, but there were by no means the only ways.
Extispicy seems remarkably different than anything in use today. The querent would take a yes or no question to a diviner. This flexible means of inquiring meant the querent could phrase the question about any topic and at any time desired, and just had to invest the sheep which the diviner would keep. The diviners did not know the content of the question and simply had to produce a positive or negative decision. They sacrificed a sheep to a god and it was therefore blessed, then only after this was it opened to study the liver. The liver was examined in a set sequence that investigated thirteen points. More than a millennium of extispicy had built up a massive amount of observational data on these thirteen spots and their positive or negative significance. If most points were positive or negative, that was the answer. For example, if six were negative seven were positive then the answer was positive; if eight were negative and five were positive then the answer was negative; and so forth. This answer was communicated back to whoever had asked and funded the extispicy.
Querent decides on a question.
Querent commissions a divination, including investment of a sheep. Diviner does not know the question.
Diviner sacrifices the sheep to a god.
Diviner opens the sheep.
Diviner studies thirteen points in a set order.
Diviner communicates if the god gave a positive or negative answer.
Extispicy was expensive. There had to be specialists versed in this huge body of literature, working under very specific questions. The querent had to offer a sheep. Most people might turn to extispicy just once or twice in their lifetime when facing tough decisions. However, the kings used extispicy to the extreme. It was the main means for the state to study possible outcomes. Neo-Assyrian reliefs depict diviners as being core parts of a war camp, alongside food preparation for soldiers and the feeding and grooming of war animals. Even when on campaign, the king took his organized institutions with him and kept the war camp tidy – reflecting his capacity as a ruler – and kept the diviners with him to make sure he made informed decisions through extispicy.
The tradition was kept continuous by texts such as Barutu and KAR 423, and liver models such as the British Museum's Old Babylonian sheep liver model. Additional organs were examined as well, but we do not know why only sheep and mostly livers were the core of this practice. Because the sheep was first sacrificed to the god and then opened, the sheep was perhaps viewed as an envelope with the god communicating a message through the organs. The liver was thus like the letter inside a bulae.
Starr, Ivan. 1990. Queries to the Sungod: Divination and Politics in Sargonid Assyria. Helsinki: Helsinki University Press.