Aquinas's foundational philosophical belief was that there's a distinction between revealed truths (things revealed to us by God) which we cannot rationally understand, and truth which we can understand (rational truths).

An avid reader of the then newly-rediscovered scientific writings of Aristotle, Thomas was rather more open to rational truths than many of his contemporaries. Indeed, like Isaac Newton a few centuries later, Thomas wanted reason and faith to sit side by side. Thus Thomas deserves credit in the history of Western philosophy for his early attempt to reconcile faith and reason, and also for incorporating elements of Arabic and Islamic thinking into his own. Averroes, or Ibn Rushd (1126-1198 AD), the great Islamic thinker, is referred to as 'the commentator' in Thomas's writings. But the hero for this Christian philosopher was the pagan philosopher Aristotle. Thomas quoted the Macedonian master more often than Christ, and referred to him as simply 'the Philosopher'.

Previous Christian philosophers, such as Augustine of Hippo (354-430), had focused almost exclusively on the next world. For example, in his book City of God, Augustine said that the perfect life can only be realised in the heavenly city that will emerge after the Apocalypse. Thomas was in principle of the same view, but notwithstanding his belief in the afterlife and heavenly bliss, he was more interested in the here and now.

Much has been written about his insights, perhaps above all his idea that God is the full actualisation of potentiality. In the history of philosophy, Thomas distinguishes himself by developing an elaborate and original metaphysics of 'being'; of what it is to exist. Inspired by Aristotle's treatise The History of Animals, Thomas proposed that all things can be categorised according to the fullness of their 'being', meaning, according to how perfectly they fulfil their potential. According to Thomas (as to Aristotle before him), all things strive to fulfil their potential; to achieve harmony between being (esse in Latin) and their inner form (essentia). In this hierarchy of nature (Thomas called it the scala naturae), God is on top as the perfect realisation of his own potential; then follow the angels; then humans; animals; plants; and at the bottom, inanimate objects. Some of this might seem dated now. The same cannot be said for Thomas's social philosophy.

Thomas wrote about international relations at a time when nations were only just emerging. He is often cited as the first thinker to develop a theory of just war. For a war to be just, there must be a just cause; it must be executed by a rightful sovereign; and the war must be waged to promote good and to avoid evil. But in truth, this was the least original part of his political theory, and it was largely derived from the Decretum Gratiani, a canon law document penned by the virtually forgotten lawyer Gratian of Bologna. Thomas's deepest insights were about domestic politics. These brimmed with often ill-disguised indignation and broadsides against corrupt politicians, "those shepherds who fatten themselves." For, "if the community is directed in the particular interest of a ruler and not for the common good, this is a perversion of government", he says in De regimine principum (On Princely Government, p.7), a short treatise he co-wrote with his colleague Ptolemy of Lucca.

In a section attributed to Thomas he was almost prescient when he wrote that, "a man who seeks personal profit from his position instead of the good of the community... is called a tyrant" (p.6). Or maybe it is just that there are timeless truths, and that there have been power-hungry men and women from Thrasymachus to Trump. But perhaps almost a millennium later certain politicians too should heed the call of the Catholic saint who wrote, "you should subject [yourself] to the same law which you promulgate" (Summa, p.139).