4 ideas
Cicero argues that the apparent success of divination comes from selective memory: people record and retell the predictions that came true while the far larger number of failed ones vanish unremarked. When a practice makes many forecasts, some will hit by chance, so a list of successes proves nothing without the count of failures.
Cicero argues that for any question, a specialist skill already gives a better answer: physicians for illness, pilots for weather, generals for war. If a matter can be known by skill, divination is unnecessary, and if it is truly a matter of chance, no art can foresee it, so divination has no work left to do.
Quintus's Stoic case runs as a chain: gods exist, gods care for humans, caring gods would give humans signs of the future, and humans can learn to read those signs; the apparent working of divination is also offered as evidence the gods exist. Cicero's reply disputes the premises and argues that the implications do not follow.
Cicero, himself an augur, admits that augury is kept up for the state's benefit and for public belief, not because it reveals truth. Seen this way, omens act as a procedural lever: officials can announce bad signs to delay assemblies or block laws. The question then shifts from whether the ritual is true to who controls it.