We hear this phrase daily: at home, in the schoolyard, in shops. ''Good boy.'' A mother just used it to praise her daughter, as the toddler solemnly put pop-up books back on a cafe shelf. ''Good girl.'' I've used it myself, perhaps thousands of times: encouraging the morning's well-eaten cereal and the evening's swiftly donned pyjamas. ''Good kids.'' The phrase is, in other words, a basic part of English-speaking child-raising; no doubt other cultures have almost identical phrases. Each is used simply: to teach a child what's ethical or, at the very least, proper.

But it is a clumsy phrase, which I avoid when I remember to; that is, when the torpedoes of child-chatter do not scuttle my best intentions.

The problem is not that the word ''good'' is incorrect or somehow inaccurate. The problem is that ''good'' is too broad and vague a word to be ethically educative. Perhaps it works well with infants and younger toddlers, but it fails as a guide for older children and teenagers. This is because ethical goodness is not simply about good behaviour. We do not see someone as good simply because they do automatically what we've trained them to do, or because they do it accidentally. Instead, to be ethical is to do the right thing in the right time and place, and to do so rationally and reliably; that is, in full knowledge that it is good.

This is why, in the modern era, we don't hold other animals to human ethical standards. We might believe a dog, for example, has a ''good'' character: loyal, gentle, clever, and so on. But we do not see Fido as a moral agent, with all the rights and privileges this suggests.

For all our animal urges and unconscious customs, we have some quantum of liberty and reason, and our morality, to say nothing of law, is prefaced on this. We are, as the technical term has it, ''moral agents''. Not because we rationally calculate every decision before we choose - we are not simply machines. Instead, rationality is what helps us to clarify our ideas and values, and to train ourselves to realise these. In other words, to be ethical is not to analyse a hypothetical before each handshake, but to freely develop the right dispositions. It is good habits, cultivated with reflection.

And this is why the ''good girl'' phrase falls flat. It helps a child perform the right acts, but not to reflect on what these acts are, or why they are valuable. For example, when my son reads a book to his little sister, he is not simply being a ''good boy''. He is being generous, patient and sympathetic. His goodness requires not only that he put aside his own books for hers but that he recognise that she cannot yet read primary school texts. In other words, he has to put himself in her shoes, and remember the feeling of exile from Enid Blyton fantasy land. My daughter is not simply ''good'' when she swings on the bars, twice her height from the tanbark: she is brave. And when she does it on bike racks over asphalt, she is not brave, but foolhardy.

These words do more than give names to what we already are. They help to think about what we might become; to give labels to the virtues we are in danger of missing or misunderstanding. Recklessness can be confused with courage, and liberality with profligacy; the words help to make our ethical aim more accurate.

Put another way, when we praise our children, we're not just encouraging good deeds. We're encouraging them to reflect on good and bad deeds, and to change as a result - perhaps even to rightfully reject their parents' values or ideas. The language is an ethical toolbox, to be used ever more artfully as we grow up.

Every good boy deserves to become … better.

Dr Damon Young is a philosopher and author. His next book, Philosophy in the Garden, will be published in December.