In his Life of Caligula Suetonius makes a distinction between the princeps and the monstrum, between the acts of a reasonable, in some respects even enlightened, ruler, and the vagaries of an erratic and cruel tyrant. The manifestations of these traits are not divided into chronological phases but depicted as anomalously present in one and the same person. Caligula’s character undoubtedly was a mixture of good and bad, and had been from the outset, but it is clear that the perception of his reign did go through a process of deterioration, and the contrast between the euphoria of its early days and the arbitrary terror of its end is undeniable. The descent into serious mismanagement and impenetrable mistrust was an ongoing process, and trying to identify a precise point where it all started to go wrong is in some respects a pointless enterprise. Philo suggests that everything went well down to the time of his illness, and began to unravel after that, and there is surely some truth in that. Dio says that he found himself in serious financial difficulties in his second year. But Josephus arguably has a better grasp of the historical period when he comments that Caligula continued to rule well and moderately for two years.1 Certainly, things seemed to take a very serious turn for the worse in AD 39. All the evidence points to it being at this time that the relations between emperor and senate, already increasingly tense, degenerated into suspicion and antipathy. But the precise nature of the conflict is far from clear, since this is also the most murky and obscure phase of Caligula’s reign. Our main source is Dio, and unfortunately his narrative is particularly confused during this period.