Short answers, pulled from the story.
De Divinatione is a philosophical dialogue written by Cicero in 44 BC about ancient Roman divination. It takes the form of a debate between Cicero and his brother Quintus, with Quintus defending divination from a Stoic standpoint and Cicero refuting it from an Academic skeptic position.
Quintus distinguishes between divination driven by skill and deduction, such as haruspicy, extispicy, augury, and astrology, and divination that occurs through dreams or frenzy, which he calls devoid of art. The first type he treats as a craft; the second belongs to a different order of experience.
Cicero argues that dreams reflect waking thoughts rather than the future. He uses his own dream of Marius during his banishment as evidence, explaining that he dreamt of Marius because he thought about him constantly while awake, not because the dream was prophetic.
Quintus cites Publius Claudius, who lost most of his Roman naval fleet in the First Punic War after ignoring auguries, Gaius Flaminius, who died with his army decimated in the Second Punic War after disregarding omens three times, and Agamemnon, who lost fleets at sea for the same reason.
Cato's saying, quoted in Book Two section 51, is: "He said he was surprised that one haruspex did not burst out laughing when he saw another one." Cicero uses it to suggest that even practitioners of divination recognized it as performance.
De Divinatione is considered one of posterity's primary sources on the workings of Roman religion and on Roman conceptions of legitimate knowledge. It is also regarded as a supplement to Cicero's companion work, De Natura Deorum.