Augustus is generally considered the first Emperor because the mechanisms that belonged to the Emperor were brought into place under his reign - usually known as the First and Second Settlements.

The First Settlement

According to Cassius Dio (53.3.1-10.7), Octavian appeared before the senate and returned his extraordinary powers to the senate and the people of Rome. Octavian claimed that (Res Gestae Divi Augusti. 34):

After this time I excelled everyone in influence, but I had no more power than the others who were my colleagues in each magistracy.

Now, is this truthful? No, not really. Octavian had the loyalty of the soldiers, an extensive network of clientelae, support amongst the urban plebs, and a lot of money. Not to mention he was also consul meaning that he still had a position of power within Rome. Senators no doubt knew that Octavian was much more powerful then they. The response by the Senate following the return to power was to ask Octavian to take control of matters and protect the state for a period of ten years; Cassius Dio writes that this was done by those in support of him, as well as by those who feared him despite knowing his true intention (53.11). Such a decision provided justification for Octavian to assume an autocratic role.

As you may know, at this point the provinces were divided into the provinces of the Roman people (provinciae populi Romani) and the provinces of Caesar (provinciae Caesaris). The basis was that the provinces of the Roman people were the provinces that had already been pacified and were at peace, and the provinces given to Octavian were those that that required pacification, which also gave him complete control over the majority of the military power of Rome (part of Octavian's speech was to declare he wants to bring peace). On paper, Octavian had lost power but could improve his position because it had been given legitimately by the senate.It was also at this point that Octavian was given the cognomen Augustus, and so was now known as Imperator Caesar divi Filius Augustus.

 

Issues Between 27 BC and 23 BC

Think of the time between 27 BC and 23 BC as a buffer period in which issues arose that poked holes between the introduction of autocratic rule and the claim of restoring the Republic (res publica restituta). Issues arose that put the new changes to the test.

The Roman elite now had to figure out what their relationship within the new system was, to which there are two examples which set it out. Firstly, M. Licinius Crassus (grandson of the famous Crassus) returned from Macedonia in 27 BC and wished to dedicate the armour of the slain enemy chieftain to Jupiter (spolia opima) which would go beyond the usual triumphal procession. Scholars suggest that Augustus blocked the move with a legal technicality on the premise that it would overshine Augustus and undermine his power. The second example is C. Cornelius Gallus. He was an equestrian and first praefectus of Egypt. According to Cassius Dio, he had success in battle and began dedicating inscriptions to himself; he was then disenfranchised by Augustus and eventually had his estate seized and he committed suicide (Cassius Dio. 53.23.5 ff.)

You then have the issues with M. Claudius Marcellus. In 24 BC, Marcellus was allowed to become aedile at 18 and stand for consul (usually at 30). This was against Republican tradition and clashed with the claim of res publica restituta. There was the issue with Marcus Primus who as governor of Macedonia had allegedly waged war against the Odrysian kingdom. According to Cassius Dio (54.3), Primus said that he attacked with the approval of Augustus, and then that of Marcellus (who had possibly died by this point). Augustus allegedly turned up at the trial and denied any involvement. Two issues arose from this. Firstly, Macedonia was a province of the Roman people in which Augustus had no authority. Secondly, Marcellus being involved promoted further idea of him being picked as a heir to Augustus, leading to suggestions of a monarchy. Dio further states due to the trial, Fannius Caeipio and others led a conspiracy against Augustus, with those involved being executed.

Another issue was that Augustus still held the position of consul continously, which effectively prevented a lot of the Roman elite from achieving the highly-sought position given only one was available each year.

 

Settlement of 23 BC

The changes during this settlement basically fixed the issues seen during the years of 27 BC - 23 BC. Firstly, Augustus stepped down from the consulship position. Whilst this did not effect his formal powers regarding the provinces of Caesar (as he had imperium proconsulare, it did reduce his power in the provinces of the Roman people. Thus, his imperium proconsulare is declared superior to other holders of imperium and became imperium proconsulare maius which allowed Augustus to command the governors of the provinces of the Roman people. Secondly, the removal of his consular powers means that Augustus did not have the ability to assemble the senate and people's senate, nor propose and pass laws. Therefore, the senate granted Augustus tribunicia potestas which gave him the powers of the tribune of the plebs without holding office, allowing him to introduce legislation and take action such as vetoing the actions of other magistrates.

 

Succession

Hence, it was these powers that future successors would be granted. M. Agrippa received imperium in 23 BC which was made maius in 18 BC in which he also received tribune powers. You have Gaius and Lucius who were consuls in 5 BC and 2 BC in order to bring them into public life. And then you have Tiberius who was given tribine powers in 6 BC, and then again in AD 4 in which he also received imperium. His imperium was raised to maius in AD 13 alongside his tribune powers being renewed.

Basically, the future emperors were granted the same powers as what was established by Augustus.

There is also the Lex de imperio Vespasiani to which I quote:

...And that whatever he considers to be in accordance with the public advantage and the dignity of divine and human and public and private intersts he shall have the right and the power to do and to execute, just as had the deified Augustus and Tiberius Julius Caesar Augustus and Tiberius Claudius Caesar Augustus Germanicus. And that by whatever laws or plebiscites it has been recorded that the deified Augustus or Tiberius Julius Caesar Augustus and Tiberius Claudius Caesar Augustus Germanicus were not bound, from these laws and plebiscites Emperor Caesar Vespasian shall be exempt; and whatsoever things it was proper for the deified Augustus or Tiberius Julius Caesar Augustus or Tiberius Claudius Caesar Augustus Germanicus to do in accordance with any law or proposed law, it shall be lawful for Emperor Caesar Vespasian Augustus to do all these things...

Vespasian was not related nor adopted by any of the Julio-Claudians and could therefore not have inherited any of their names. What his law did was grant the same powers that they had to Vespasian, and thus we begin to see the usage of Emperor (imperator), Caesar, and Augustus as titles rather than names. Given that this all began with Augustus, it is why he is considered the first emperor.

Thank you for the great explanation! I can definitely see why people would say that he would be the first emperor.

However, I cannot help but feel that this is a title we have given him afterwards. To my knowledge, Augustus never referred to himself as being emperor or ruler of Rome, since that would have brought him in a difficult position, as Caesar's death would have taught him. Tiberius was also described (by Suetonius I believe) to be reluctant to inherit Augustus' position.

Therefore, I would suggest that it would be more useful to refer, at least to Augustus, as princeps, since I don't think Augustus had the security to call himself emperor.

Do you agree or no?