Joe Biden’s inauguration speech was not big on surprises – but his decision to quote St. Augustine of Hippo (354-430) was certainly one of them.

After condemning the recent unrest in Washington, Biden called on Americans to set their differences aside, and used a passage from Augustine’s De civitate Dei (19.24) to explain what, in his view, unites them. ‘Many centuries ago’, the new president observed, ‘Augustine… wrote that a people was a multitude united by common agreement about the objects of their love.’

To be sure, it was not an exact quotation. But as a statement of the new president’s approach to government, it is nevertheless hugely significant.

To understand why, we have to go back to the Roman statesman and philosopher, Marcus Tullius Cicero (106 BCE – 43 BCE).

In his dialogue, De re publica, Cicero argued that a ‘people’, as distinct from a mere mob, could be defined as ‘a multitude united in agreement about justice and a community of interest’ (1.25.39). On this basis, he attempted to distinguish between legitimate and ‘corrupt’ forms of government based on their adoption (or repudiation) of law and the common good.

This all sounded perfectly sensible. But as Augustine spotted, almost five centuries later, there was a problem. What does ‘justice’ mean? Although Cicero had tried to contrast it with self-interest, even he had acknowledged that not everyone sees it in the same way. What you think of as ‘just’ might not seem ‘just’ to me.

For Augustine, such disagreement was only to be expected. ‘True’ justice, he believed, belonged to God alone. When Adam and Eve were expelled from Eden, mankind was cut off from it for ever. At the same time, the knowledge of sin also divided humanity into two irreconcilable groups – or ‘cities’. Those who belonged to the ‘earthly city’ were sinners who loved only themselves; while those who were members of the ‘City of God’ were driven by a love of others and the divine (De civ. Dei 14.1). Since each of these groups was defined by a different form of ‘love’, it was inconceivable that they would ever agree about what was ‘just’. And since no-one but God could know who belonged to which ‘city’, this meant that, if a society was governed with some imperfect understanding of ‘justice’ in mind, it would invariably split apart – exactly like the Roman Republic.

But this left Augustine with a question: how should a ‘people’ be defined? And how should it be governed?

Fortunately, a solution was at hand. In the De civitate Dei, Augustine argued that, even if we don’t all share the same sort of love, there are still a few things we all hanker for. Regardless of which ‘city’ a person belongs to, they will still place a high value certain basic necessities, such as (temporal) peace, security, a good home, freedom from want and so on. Of course, not everyone will desire them for the same reason. Take peace. Whereas a member of the ‘earthly city’ might enjoy peace for its own sake, revelling in the opportunity for self-gratification in the here and now, a member of the ‘City of God’ might use it to pursue virtue and merit salvation in the next life. But this doesn’t alter the fact that everyone wants – or ‘loves’ – peace. Everyone.

According to Augustine, this shared desire for a few, basic necessities is the glue which holds society together. It was for this reason that, in the De civitate Dei, he chose to reject Cicero’s definition, and instead characterised a ‘people’ as a ‘multitude of rational creatures bound together by a common agreement as to the objects of their love.’ (19.24)

As Augustine recognised, this had important implications. If a people is defined by the objects of their love, it is the government’s task to assure the basic needs they desire, rather than aim for a divisive and unattainable ideal of ‘justice’. Each person will, of course, either ‘use’ or ‘enjoy’ those necessities as they see fit; but all can be certain that they will be able to pursue their own ends – and crucially, society will hold together.

This is what makes Biden’s decision to quote Augustine so telling. Even if he and his speechwriters are unlikely to have had the details of Augustine’s disagreement with Cicero in mind when drafting the speech, the bishop of Hippo’s definition of society neatly encapsulates the essence of the president’s message.

Donald Trump’s presidency has left deep divisions in American society. There are still groups who doubt the legitimacy of the election, who harbour wild conspiracy theories, and who favour bullets over ballots. In such a situation, President Biden could perhaps be forgiven for feeling the attraction of Cicero’s definition of society and administering harsh ‘justice’ to the enemies of democracy. But that would only worsen divisions – and potentially lead to further acts of violence.

Augustine’s definition of society points towards an alternative. President Biden made it clear that he will be ‘a President for all Americans’. And that means tackling those things that everyone holds dear: peace, affordable healthcare, a good future for our children. It might not satisfy all of his supporters (especially those baying for the impeachment of his predecessor), but it is perhaps the best chance of bringing a shattered nation together – and restoring America’s hope in the future.