"Evil" is a strong word, and a provocative one. Nowadays it tends to be reserved for acts of exceptional cruelty: the Moors murders, organised child abuse, genocide. It is not just the extreme nastiness of such acts – and their perpetrators – that makes people describe them as evil. There is something unfathomable about evil: it appears to be a deep, impenetrable darkness that resists the light of reason. To say that a murderer has killed because she or he is evil is really to point to an absence of motive. Far from the usual muddle of human motivation, evil has a cold, horrifying purity. Phrases like "unthinkable evil" or "unspeakable evil" highlight the way the word is used to say the unsayable, to explain the inexplicable.

So how can we think about evil? Perhaps we can't, or shouldn't. Ludwig Wittgenstein famously wrote that we should remain silent about "that whereof we cannot speak" – a quotation beloved of lesser philosophers seeking a convenient way to end an academic paper. On a more practical level, most victims of evil will find that simply coping takes all their energy – and in the midst of their suffering, it may be difficult to disentangle the questions "why?" and "why me?" But the very familiarity of these questions suggests that there is something about evil that calls for thinking. And Wittgenstein's remark about remaining silent can be countered by Martin Heidegger's suggestion that the proper subject matter for philosophical thinking is precisely what is "unthought" and even unthinkable.

The Christian tradition offers huge resources for our thinking about the nature, origin and meaning of evil. This is partly because the history of western philosophy is intimately bound up with Christianity, so that supposedly secular debates on morality and human nature usually involve theological ideas even if these remain implicit. But more specifically, the Christian doctrine of creation makes the question of evil particularly pressing. If the world was designed and brought into being by a perfectly good, just and all-powerful creator, why does it contain evil at all? If God did not create evil, where did it come from? And why would God make human beings capable of extreme cruelty?

In this religious context, the concept of evil becomes elastic, encompassing much more than pure wickedness. The Christian believer comes under pressure to explain the existence of various kinds and degrees of suffering, unpleasantness, deviation and disorder. In their discussions of the "problem of evil", theologians have distinguished between the "moral evil" of human wrongdoing and the "natural evil" wrought by destructive events like earthquakes and tsunamis. They have also had to worry about the apparently imperfect conditions of life itself – our mortality, our finite knowledge, and our limited power – which are sometimes described as "metaphysical evil". Why didn't God just make everything better?

David Hume gave a pithy summary of the problem of evil in his 1779 Dialogues Concerning Natural Religion: "Is God willing to prevent evil, but not able? Then he is impotent. Is he able, but not willing? Then he is malevolent. Is he both able and willing? Whence then is evil?" For many critics of theistic religion – and Hume can be included among them – this is not a problem to be solved, but a basic and definitive objection to belief in a creator God.

The obvious secular response to Hume's (rhetorical) question is to simply accept that the world is as it is, evil and all. This view might be developed by pointing out that nature, having been formed through a process of evolution involving a highly complex web of "survival mechanisms", is morally neutral. Or it might be argued that "evil" is just a word that human beings use to describe certain aspects of life, and that it is only our ethical or religious idealism that makes evil seem problematic.

These ideas are certainly worth exploring, but even from a non-religious perspective they need not put an end to questions about evil. Even if the Judeo-Christian doctrine of creation is unacceptable as a quasi-scientific account of the origins of the world, it has a religious significance that reaches beyond this. Creation gives expression to a fundamental view shared by many believers and non-believers: that the world is good, that life is worthwhile, that it is better to be than not to be. (The practical alternative, after all, could be suicide.) Although this belief is often inarticulate, it seems to underlie the sense of violation that arises in response to radical cruelty and injustice. The idea that certain human rights ought not to be infringed – that these are "sacred", even – implies not moral neutrality, but faith in a basic standard of decency. The outrage and protest frequently voiced in response to evil point to a deeply-held belief in norms of goodness and justice.

The religious idea that thinking about evil involves coming to terms with a darkness in all our hearts – traditionally understood as sin – also provides food for thought. If the tabloid concept of evil is the visible tip of an iceberg, signalling a submerged mass of more widespread psychological tendencies, then the effort to understand evil responds not to any theological dogma, but to Socrates' philosophical imperative: "Know yourself!"

The Christian tradition of theodicy – the effort to defend God, and therefore also the world's fundamental goodness, in the face of evil – links to questions about the meaning, purpose and value of life that touch us all. Some atheistic responses to suffering and meaninglessness, such Friedrich Nietzsche's reflections on tragedy or Albert Camus's "existentialist" interpretation of the ancient myth of Sisyphus, stand directly in this tradition. At the end of this series we will consider whether religious and philosophical thinking on evil remains relevant when scientists and psychiatrists are producing compelling accounts of cruelty. Next week, though, we will turn to a 4th-century Christian theologian whose ideas about evil provoked centuries of debate both inside and outside the church.