From a distance, few areas of knowledge seem more enticing or more profound than philosophy. In a secular age, philosophy can look like the ultimate authority on life's great questions, the natural place to seek answers to the riddles of human unhappiness. Philosophers, like rocket scientists, look as if they have access to some very complex and important truths. But despite an enticing exterior, modern philosophy often disappoints those who study it more closely. Issues that seem so urgent to many contemporary theorists and philosophers (What is the signifier? What is the subject?) don't often echo our own priorities (Why am I so shy? How can I be happy?).

This may explain why, in universities across Britain, more students are now enrolling in the study of ancient philosophy (Plato, Aristotle, the Hellenistic schools) than in the study of theorists such as Derrida, Foucault, Baudrillard and Deleuze. Ancient philosophy remains far more faithful to most people's idea of what philosophy should be about. The ancient philosophers believed quite simply that philosophy should in some way help to change one's life for the better - a beautiful ambition almost entirely absent from modern philosophy (and relegated instead to the problem pages of magazines and afternoon chat shows). "Any philosopher's argument which does not treat human suffering is worthless. For just as there is no profit in medicine if it does not expel the diseases of the body, so there is no profit in philosophy either, if it does not expel the suffering of the mind." These are the words of Epicurus, born on the island of Samos, a few miles off the Ionian coast, in 341BC. They also happen to reflect the aspirations of most students preparing to study philosophy at university, aspirations sadly shared by almost none of their lecturers. In ancient philosophy, we find a repository of the therapeutic ideals which most of us still associate with the subject, but which have largely disappeared from the modern curriculum. To listen again to Epicurus's exhortations: "Let no one put off studying philosophy when he is young, nor when old grow weary of its study. For no one is too young or too far past his prime to achieve the health of his soul. The man who alleges that he is not yet ready for philosophy or that the time for it has passed him by, is like the man who says that he is either too young or too old for happiness."

Philosophy is best defined not so much by its subject matter, as by its method of inquiry: logical, syllogistic and axiomatic. Many scientific subjects which became independent disciplines began life as branches of philosophy (up until the 19th century, physics courses in universities were described as natural philosophy). Over the long history of philosophy, there have been five areas in which the majority of practitioners have done their thinking: epistemology, ethics, political theory, aesthetics and the philosophy of religion.

Though the first of these branches regularly puts most people off philosophy, it occupies the dominant position in the modern curriculum. Ethics interests the majority of people, and was of the greatest concern in ancient times. The Hellenistic schools of Greece and Rome - the Epicureans, Sceptics and Stoics - were passionately committed to the idea that philosophy should address the painful practical problems of human life - death, love, sexuality and anger.

To take an example, confronted with someone who was worried about death, an Epicurean would break the problem into components, arguing that the only things we should fear were those that caused us pain. When dead, we would feel neither pain nor pleasure. Therefore, there was no reason to fear death. "The man who has truly comprehended that there is nothing terrible in ceasing to live, has nothing terrible to fear in life," concluded Epicurus.

The dialogues of Plato similarly engage with the eternally important questions. Few philosophers have had a more practical view of the thinking life. For a start, it wasn't necessary to disengage from ordinary commitments: philosophising could go on alongside shopping, working, bathing, loving; it was no alternative to an active life - rather, its necessary complement. The point was emphasised by Plato's decision to develop Socrates' thoughts within dialogues set in quasi-novelistic contexts. The central tenets of western philosophy were shown to unfold naturally during conversations between a man who didn't wash his cloak too frequently and some of his friends on their strolls to the harbour and visits to the gymnasium in fifth-century BC Athens. The dialogues were strewn with banter and gossip unexpected in philosophical treatises: such static belonged to existence, and thus philosophy, its illuminator, had a duty not to shy from it.

As Charmides opens, we find Socrates, just returned from the siege of Potideae, catching up with friends at the wrestling school opposite the temple of Basile, south of the Acropolis. They talk about the battle, then the subject turns to a young man called Charmides, said to be extraordinarily pretty, who is on his way to the wrestling school. Socrates recounts his arrival:

"Charmides came, and he caused a great deal of laughter: each of us sitting down tried to make room for him by pushing his neighbour away in a frantic attempt to have the boy sit next to him, until we forced the man sitting at one end of the row to stand up and tipped the man at the other end off sideways. In the event Charmides came and sat between me and Critias. Well, by then, my friend, I was in difficulties, and the self-assurance I'd felt earlier . . . had been knocked out of me . . . That was the moment . . . when I saw what was inside his cloak. I was on fire, I lost my head . . ."

Philosophy should not imply its emergence from a vacuum, suggests Plato; it is anchored in a world in which heads will be lost after glimpses inside others' cloaks.

Though we always associate the new with the superior, the ancient philosophers provide a humbling lesson. It seems that the earliest thinkers may have had a better idea of philosophy's task than later ones. It is even more humbling for the philosophical establishment to find that students are discovering all this, forcing a change to the way the subject is taught by drifting spontaneously to the consoling company of Plato and Epicurus.

Alain de Botton is the author of "How Proust Can Change Your Life", Picador, £5.99