Religions, despite what people think of as their personal consolations, have not served the world well. They create division and conflict, they impose unlivable moralities of denial and limitation, and they demand that we think of the world as our remotest ancestors did, thousands of years ago. The cry raised by defenders of religion is: but what would you put in its place as a view of the world by which we can live? The answer is: something far better, deeper, kinder and warmer – and far more rational – namely: humanism.

Humanism is a non-religious ethical outlook based on an interest in human affairs at the human scale. It is not a doctrine or a set of rules; it is a starting point, its founding idea being that ethics must be based on the facts of human experience. For some, the result of thinking for themselves about ethics might be close to a conventional moral outlook; for others, the result might be less conventional. Either way, there are just two constraints: that one's choices must not be aimed at harming others, and that one must be able to make a solid case for one's outlook if challenged by others.

One great flaw in religion-based moralities is that they are not thought through and chosen on the basis of individual responsibility, but are imposed from outside in a one-size-fits-all way. As a result, religious morality too often cuts across the grain of human nature, distorting it and crushing its natural impulses, not least as regards sex – always an inflated subject of interest for religious moralists, who throughout history have been frightened of it and bent on limiting it as strictly as possible.

As this shows, humanism is a response to Socrates's invitation to live the chosen life, rather than a life prescribed by doctrines inherited from the traditions, especially the religious traditions, of whatever community one happens to be born into. Religious moralities assume that there is one great truth and one right way to live for everyone. Another great flaw with religious morality is that it says if you do not obey, you will be punished. The threat of punishment is not a logically adequate ground for moral behaviour, even if it is prudent to avoid punishment by behaving as ordered. Unless one's moral outlook comes from being thought-out and chosen for oneself, it is at best an imitation of morality, at worst a subversion of it.

The foundation of a humanist ethic is that it has to start from our best understanding of human nature and the human condition. The "human condition" is somewhat easier to describe than "human nature", that complex thing which literature, psychology, philosophy and individual experience all struggle to understand. Whereas a study of history and a thoughtful reading of literature together offer abundant insights into the human condition, the sheer diversity in human nature makes the task of understanding it a work that could demand whole lifetimes as we seek to make sense of ourselves and others, especially the others we care about.

But the effort to understand human nature is itself constitutive of what makes a good and worthwhile life. It is easy to prove this: consider the opposite, namely, a life lived in carelessness and indifference towards the question of who we are and how we can best relate to others. What a waste that would be. In attempting to understand humanity we can expect to find that what motivates people is, too often, not very admirable and sometimes downright appalling. But this is not the majority story. In every village, town and city in the world, every minute of each day, there are millions of acts of ordinary co-operation, courtesy and kindness, and they constitute the majority of human interactions.

An important assumption that humanism makes is that people are, or at least can be, self-creating and self-determining. But, in many cases, the burden of history and society makes self-creation impossible. This certainly happens when people are trapped in a religious tradition which tells them what to think and how to behave, and refuses to allow them freedom.

But the effort to be a free-minded individual in pursuit of worthwhile goals suited to one's individuality is surely central to the very idea of the good: it is what gives us our best chance to be fully human, and at the same time – in the spirit of shared humanity – to develop our affections in our communities, to promote the values of kindness and tolerance, and to celebrate the enjoyment of all the things that make life beautiful and satisfying.

Because humanism draws on 2,500 years of non-religious ethical thinking since Socrates, it is a deep, rich tradition of insight, wisdom and inspiration, and it is this without any supernaturalistic beliefs involved. That means that it offers the possibility of truly global ethics that everyone could live by. Consider a utopia in which people, having been liberated from religion at last, can agree to base their ethics on a generous view of human nature and needs.

AC Grayling's The God Argument is published on 14 March