In 23 B.C. he gave up his first tentative plan and made thetribunicia potestas, instead of the consulship, which he resigned on June 27, the second pillar of his power. The tribunician power was his for life, but he now made it annual as well as perpetual, and dated from this year the years of his reign. Thus in a very narrow sense the Empire might be said to have begun in 23 B.C.; in that year at least the constitution of Augustus received its final form. After this year, his eleventh consulship, Augustus held that office only twice (5 and 2 B.C.). Subsequent Emperors generally assumed it more than once; but it was rather a distinction for the colleague than an advantage for the Emperor
But the tribunicia potestas alone was not a sufficient substitute for the consulare imperium which Augustus had surrendered by resigning the consulate. Accordingly a series of privileges and rights were conferred upon him by special acts in 23 B.C. and the following years. He received the right of convening the senate when he chose, and of proposing the first motion at its meetings (ius primes relationis). His proconsular imperium was defined as “superior” (maius) to that of other proconsuls. He received the right of the twelve fasces in Rome, and of sitting between the consuls, and thus he was equalized with the consuls in external dignity (19 B.C.). He probably received too the ius edicendi, that is, the power of issuing magisterial edicts. These rights, conferred upon Augustus by separate acts, were afterwards drawn up in a single form of law, by which the senate and people conferred them on each succeeding Emperor. Thus the constitutional position of the Emperor rested on three bases : the proconsular imperium, the tribunician potestas, and a special law of investiture with certain other prerogatives.
The title imperator expressed only the proconsular and military power of the Emperor. The one word which could have expressed the sum of all his functions as head of the state,—rex—was just the title which Augustus would on no account have assumed; for by doing so he would have thrown off the republican disguise which was essential to his position. The key to the Empire, as Augustus constituted it, is that the Emperor was a magistrate, not a monarch. But a word was wanted, which, without emphasizing any special side of the Emperor's power should indicate his supreme authority in the republic. Augustus chose the name princeps to do this informal duty. The name meant the first citizen in the state—princeps civitatis—and thus implied at once supremacy and equality, quite in accordance with the spirit of Augustus' constitution; but did not suggest any definite functions. It was purely a name of courtesy. It must be carefully distinguished from the title princeps senatus. The senator who was first on the list of the conscript fathers, and had a right to be asked his opinion first, was called princeps senatus; and that position had been assigned to Augustus in 28 B.C.. But when he or others spoke or wrote of the princeps, they did not mean “prince of the senate”, but “prince of the Roman citizens”. The Empire as constituted by Augustus is often called the Principate, as opposed to the absolute monarchy into which it developed at a later stage. The Principate is in fact a stage of the Empire; and it might be said that while Augustus founded the Principate, Julius was the true founder of the Empire.
According to constitutional theory, the state was still governed under the Principate by the senate and the people. The people delegated most of its functions to one man, so that the government was divided between the senate and the man who represented the people. In the course of time the republican forms of the constitution and the magisterial character of the Emperor gradually disappeared; but at first they were clearly marked and strictly maintained. The senate possessed some real power; assemblies of the people were held; consuls, praetors, tribunes, and the other magistrates were elected as usual. The Principate was not formally a monarchy, but rather a "dyarchy", as German writers have called it; the Princeps and the senate together ruled the State. But the fellowship was an unequal one, for the Emperor, as supreme commander of the armies, had the actual power. The dyarchy is a transparent fiction. The chief feature of the constitutional history of the first three centuries of the Empire is the decline of the authority of the senate and the corresponding growth of the powers of the Princeps, until finally he becomes an absolute monarch. When this comes to pass, the Empire can no longer be described as the Principate.
The Princeps was a magistrate. His powers were entrusted to him by the people, and his position was based on the sovereignty of the people. Like any other citizen he was bound by the laws, and if for any purpose he needed a dispensation from any law, he had to receive such dispensation from the senate. He could not be the object of a criminal prosecution; this, however, was no special privilege, but merely an application of the general rule that no magistrate, while he is in office, can be called to account by anyone except a superior magistrate. Hence the Princeps, who held office for life and had no superior, was necessarily exempted from criminal prosecution. If, however, he abdicated or were deposed, he might be tried in the criminal courts. And as Roman Law permitted processes against the dead, it often happened that a Princeps was tried in the senate after his death, and his memory condemned to dishonor, or his acts rescinded. The heavier sentence deprived him of the honor of a public funeral and abolished the statues and monuments erected in his name; while the lighter sentence removed his name from those Emperors, to whose acts the magistrates swore when they entered on their office. When a Princeps was not condemned, and when his acts were recognized as valid, he received the honor of consecration.
The claim to consecration after death was a significant characteristic of the Principate, derived from Caesar the Dictator. He had permitted himself to be worshipped as a god during his lifetime; and though no building was set apart for his worship, his statue was set up in the temples of the gods, and he had a flamen of his own. After his death he was numbered, by a decree of the senate and Roman people, among the gods of the Roman state, under the name of divus Julius. His adopted son did not venture to accept divine worship at Rome during his lifetime; he was content to be the son of a god, divi filius, and to receive the name Augustus, which implied a certain consecration. But like Romulus, to whom he was fond of comparing himself, he was elevated to the rank of the gods after his death. It is worth observing how Augustus softened down the bolder designs of Caesar in this as in other respects. Caesar would have restored royalty without disguise; Augustus substituted the princeps for the rex. In Rome, Caesar was a god during his lifetime; Augustus the son of a god when he lived, a god only after death.
In one important respect the Principate differed from other magistracies. There was no such thing as designation. The successor to the post could not be appointed until the post was vacant. Hence it follows that, on the death of an Emperor, the Empire ceased to exist until the election of his successor; therepublic was in the hands of the senate and the people during the interim, and the initiative devolved upon the consuls. The principle “The king is dead, long live the king”, had no application in the Roman Empire.
As a magistracy, the Principate was elective and not hereditary. It might be conferred on any citizen by the will of the sovereign people; and even women and children were not disqualified by their sex and age, as in the case of other magistracies. Two, or rather three, acts were necessary for the creation of the Princeps. He first received the proconsular imperium and along with it the name Augustus; subsequently the tribunician power; and also other rights defined by the special Law de imperio. But it must be clearly understood, that his position as Princeps really depended upon the proconsular imperium, which gave him exclusive command of all the soldiers of the state. Once he receives it, he is Emperor; the acquisition of the tribunician power is a consequence of the acquisition of the supreme power, but is not the supreme power itself. The day on which the imperium is conferred (dies imperii) marks the beginning of a new reign.
It is important to observe how the proconsular power was conferred on the Princeps. It was, theoretically, delegated by the sovereign people, but was never bestowed or confirmed by the people meeting in the comitia. It was always conferred by the senate, which was supposed to act for the people. When the title Imperator was first conferred by the soldiers, it required the formal confirmation of the senate, and until the confirmation took place the candidate selected by the soldiers was a usurper. On the other hand the Imperator named by the senate, although legitimate, had no chance of maintaining his position unless he were also recognized by the soldiers.
The position of the new Princeps was fully established when he was acknowledged by both the senate and the army. After Augustus, the proconsular power of the Princeps was perpetual, and it was free from annuity