The Buddha's diagnosis of the human condition is penetratingly simple: existence is marked by suffering, impermanence, and the absence of an abiding self. Like Heraclitus, Buddha thought that nothing ever truly is, but becomes. The greatest affliction is suffering, which stems from our craving for the many worldly things we mistakenly take to be true goods.
Like a skilled physician of the mind, the Buddha prescribes a course of therapy: follow the noble eightfold path, cultivate wholesome states, and practice virtuous action. Sooner or later, through dedicated effort, one arrived at a state of insight into the woes of the human condition, and is thereby released from the dreadful cycle of repeated birth.
For a treatment to work, however, it is enough that it tends to the proximal causes of the disease. And yet, the Buddha is not satisfied with curing the patient's disease: he intends to eradicate disease itself from the world. To this end, he proposed as the culmination of his teachings the achievement of Nibbana, the highest good that brings the cessation of desire and associated suffering. It is appropriate action, guided by the wisdom afforded by Nibbana, that gives no rise to further karma – which is no more than the law of action and is consequences. As the karmic chain that culminated in our birth works itself out, no more life shall arise: rebirth in Buddhism is not the transmigration of a soul, but the transmigration of ignorant action and the inheritance of its consequences by other living beings.
The assumption here is that life itself is an evil so great that great care must be taken to not act in such a way that our actions could cause more life to take form, even indirectly. His prescription is, indeed, that one should permanently retreat from worldly affairs and live one's life in a monastery, where a life of meditative absorption will eventually lead to release from the cycle of birth and death. Life's joys are temporary, and are therefore not worth our efforts; Nibbana is permanent insight into the nature of existence, and it alone is worth our efforts.
Is life a great evil? It is true that life is permeated by sorrows small and great, as well as by joys that are all too fleeting. Our minds hold to pain more easily than to pleasure, and there is no end to the drama of existence. But the facts of life do not make it evil, and the stage of human drama is a possibility for transformation, beauty, and goodness. Other traditions such as Stoicism also see the contingencies of life and its impermanent nature, and advise virtue and prudent action without the need to seclude oneself from the world – quite the contrary. It is in the world, full with temptations and obstacles, that we must face the challenges of living well.
Other questions remain, such as whether the meditative method the Buddha proposed is reliable – that is, can the mind be trusted to investigate itself? Later Mahayana traditions go further than this, and claim that meditative practise will reveal the nature of reality as it truly is. This is an extraordinary claim that seems to ignore the fact that we do not perceive reality as it truly is, but rather what our sensory equipment allows us to perceive, and our mental dispositions allows us to conceive. This could call the very goal of Buddhism into question.
There is little doubt that the Buddha has made an accurate diagnosis of unwholesome desires and its consequences, both in the short and long term. But it seems to me that an effective and humane therapy of desire must cure us of our false beliefs by offering a firmer ground than self-interest and the condemnation of all human joy for a freedom that, perhaps, is not worth having. The oft-maligned Aristippus (the hedonist, no less) would have told Siddharta what he told his critics: “It is not abstinence from pleasures that is best, but mastery over them without ever being worsted.”