Perhaps the most important form of divination in Rome was haruspicy (which spell-check insists is not a word, but is). Performed by a haruspex, haruspicy was the art of determining the will of the gods by examining the entrails of animals – particularly sacrificed animals and most commonly (but not exclusively) the liver. The most common thing haruspicy might tell you is if the sacrifice was accepted: a malformed or otherwise ill-omened liver might indicate that the ritual had failed and that the god had refused the sacrifice.
Remember that the do ut des system is essentially one of bargaining with the gods, and the god you are bargaining with always has the option of simply refusing the bargain. This might mean some failure in the mechanics of the ritual (necessitating it be performed again), or that the god had been offended in some way, but it might also mean something more. A lot of sacrificial rituals were done at the outset of important tasks – before battles, political events, etc. What the god might be telling you then with a failed sacrifice is “DO NOT PROCEED”.
The practitioner is given a bit of wiggle room on how to interrupt a failed sacrifice in this way: it might mean “don’t attack at all”, but it might also mean “don’t attack now”. Roman generals, ready to attack, might repeat the same ritual over and over again, like a runner at the start of a race waiting for the “go” signal.
But more information was potentially available, because the exact nature of the liver and its quality might signal more things. In Rome, it was understood that the very best knowledge in this regard came from the Etruscans (an example of how antiquity lends credibility to ritual – Etruscan religion was old even to the Romans, and thus had acquired a strong reputation). The reading of a liver could be complex: we find “liver models” from both Italy and the Near East with guidance on how to interpret different parts of the liver of a sacrificed animal. This could be fairly specific: famously, it was haruspex who warned Caesar about the danger of the Ides of March (Seut. Caes. 81.2).
Another key system for divining the will of the gods in Rome was augury, the reading of the flights of birds (mostly, there are actually other categories of auspicia); doing so is called taking the auspices, and the men who do so are the augurs. Augurs were particularly important in political matters, taking the auspices for elections and the like. Unfavorable auspices could invalidate even a consular election: the gods get a vote too.
Bret Devereaux, “Collections: Practical Polytheism, Part III: Polling the Gods”, A Collection of Unmitigated Pedantry, 2019-11-08.
