There are many elements of Gautama's philosophy that are compelling and useful, but some of his main ideas are antiquated and riddled with irreconcilable contradictions (such as soulless rebirth and blissful equanimity). It's worthwhile separating what he got right from what he got wrong.
What did the Buddha get wrong?
Here are six specific problems:
1. The centerpiece of Gautama's teaching is a method for penetrating reality deeply enough to halt the cycle of sequentially unsatisfying existences that we are all captive to. The notion of rebirth into a new life, the form of which is influenced by the intentions and actions of a former existence (karma), is fanciful cosmology that is impossible to make intelligible in any coherent or convincing way. Making the idea even more problematic was Gautama's insistence that there is no fixed soul to migrate from a dead body to a new one: just a vague, mysterious flicker of energy jumping from one body to the next. This flame-like energy somehow carries the seeds of destiny with it such that bad deeds are transformed into more misery and good deeds into less. The only way to break the cycle of rebirth is to extinguish the flame (nirvana) by renouncing all desire. Not exactly the stuff of 21st century physics.
2. Gautama required detachment from ALL earthly pleasures. Passion, arousal and excitement are just as distracting to the project of enlightenment as anger, frustration and sorrow. The only suitable emotion is equanimity (upekkha), the serene neutrality that accompanies complete detachment and engenders the "bliss" (piti) of enlightenment, a permanent serenity that Gautama said was indescribable to anyone who hasn't experienced it. His rejection of all earthly pleasure is severe, stark and sterile. It's a high-stakes gamble that requires huge sacrifice based on the assumption that ultimate and permanent serenity is possible by following his eightfold path.
3. There is no place in Gautama's philosophy for individuality, which merely reinforces the illusion of self that traps us in the cycle of painful lives: no place for striving, ambition, accomplishment or any sense of personal competence or pride. Gautama's "middle way" was uncompromising: the idea of pursuing individual meaning to enrich our lives is anathema because there is no meaning to be had, other than awakening to the truth that any search for meaning is destined to fail, bringing disappointment and depression. This complete rejection of personal identity and individual goals imposes a horrendous limitation on an individual's life. While our ambitions and dreams do contribute to our angst, they also provide us with a unique source of energy, motivation and fulfillment.
4. Because enlightenment is only possible to those who fully renounce everything, nirvana is only available to monks. Enlightenment is not in the cards for someone who is distracted by earning a living or raising a family and will therefore be destined to suffer and reborn into more suffering. The lay person can still benefit from a meditative practice, but only insofar as she can lessen her misery in the next life cycle.
5. While Gautama advocated paying alms to monks to support their work in achieving nirvana, he was silent on the issue of helping the less fortunate. The poor and downtrodden are presumably living out the infected karma of their past lives. Monks are doing the noblest work possible, so their sustenance requires support from laity, whose generosity will be rewarded with a slightly less miserable existence in the next life.
6. Gautama was ambivalent about the role of women: it wasn't until one of his followers (Ananda) pleaded with him that he reversed his obstinate objection to ordaining women, acquiescing to allow his aunt to enter his Order. Even then, he imposed eight strict restrictions on nuns, such as the requirement that they must always stand when in the presence of a male monk. In his dying days, Gautama instructed the same disciple Ananda to not speak to women. Scholars are not certain if these accounts of misogyny were inserted into his philosophy later by less liberal-minded followers; nonetheless, they are consistent with Gautama's view that feelings of romance and lust undermine a monk's ability to become enlightened. (Note that he abandoned his wife and son in his quest for enlightenment.)
What did the Buddha get right?
If we expunge the ancient mythology, the contradictory ideas and incoherent cosmology from the Buddha's sermons, are we left with anything useful? Absolutely - especially these six:
1. The contingency of existence: he insisted that everything about our lives could be otherwise. A long history of causal factors have combined, often randomly, to create the present, fleeting moment. There is nothing completely reliable or predictable about the next moment that follows this one, except that it will be connected to the present by a network of largely invisible, interacting causal factors. When we forget how contingent all aspects of life are, our expectations become unrealistic: we risk both setting ourselves up for disappointment and taking good fortune for granted.
2. Related to this notion of fundamental contingency is the view that everything is interconnected. Systems of inter-related causes define everything around and within us. This is the Buddha's concept of dependent origination, such that "a first beginning cannot be discerned." His observation was a profound insight about the causal complexity that swirls around us, which we habitually oversimplify in our facile explanations of life's most demanding challenges, including and especially our difficult interactions with other people. The Buddha anticipated complexity science and systems theory by millennia.
3. Not only are we oblivious to the causal complexity that we live within, but Gautama maintained that much of what we consider factual is pure conceptual fabrication, representing a barely tenable connection with the real world. He taught that we construct our world view by adding all kinds of invented thought to our immediate perceptions, based on our personal histories. He instructed that we should limit our thinking to our immediate sensory experience (our "sensorium") since anything outside of immediate experience is meaningless. While he stretched the point, he anticipated the British empiricist philosophers by two millennia with his insight that the further we travel from immediate experience, the higher the probability of conceptual error. He admonished us to avoid struggling with metaphysical conundrums such as "Why are we here?" and "Is there life after death?" because they are distracting to our serenity and because the only meaning that is available to us is our moment-to-moment experience. To believe otherwise is like a fish swimming in the ocean in search of water.
4. His description of self as an unfolding process in a constant state of flux was fairly sophisticated. His notion of the fluidity of identity and no fixed self, although extreme, anticipated existentialist philosophy by two millennia. Notwithstanding some aspects of our temperament that are innate, our personalities, values and ambitions are indeed always shifting and evolving.
5. With his "two darts" analogy, Gautama illustrated our tendency to inflict unnecessary pain on ourselves by adding a problem to a problem. He describes a person whose skin is pierced by a painful dart, and then grieves and laments the misfortune and discomfort. It's as if the person has been struck by a second dart: the first one caused bodily pain and the second one caused mental anguish in addition to the original injury. A person who understands the true nature of reality avoids the second dart: he will not wallow because he does not expect to live a life of uninterrupted pleasure. The enlightened person is never attached to the feeling of comfort so he greets his discomfort with equanimity, rather than self-pity and indignation. We do indeed inflict incremental anguish on ourselves: the traffic jam that we treat like a catastrophe, the friendly criticism that we are outraged by, and the myriad inconveniences that we perpetually over-react to, and torture ourselves (and others) with. We often respond to the world as if life was custom-made for our comfort.
6. Probably his greatest lesson of all was his distinction between "skillful" and "unskillful" thinking, which differentiates thought patterns that are productive from those that are damaging. He anticipated the 20th century development of cognitive therapy when he pointed out that awareness of our own thinking is key to being able to intervene when unskillful thoughts take hold. Our thoughts ought to be objects of thought themselves, so that we can protect ourselves from the destructive thinking that undermines our effectiveness and happiness. Meditation bolsters our skillful thinking while limiting unskillful thought patterns; it empowers us with the insight and control to manage our frenetic minds and reduce the suffering we inflict on ourselves.
The Buddha was certainly not the first meditator. But he may have been one of the very first cognitive therapists. Even though his grander aim was a permanently transformed consciousness (that escapes the cycle of rebirth), his method of replacing illusory, misguided thinking with a more accurate view of reality mirrors the cognitive therapeutic approach conceived in the 1960s. We don't have to endorse his lofty cosmological goals to benefit from the self-reflection that he recommended, as a means of freeing ourselves from being hostage to our destructive thoughts. And we certainly don't have to accept his all-encompassing rejection of earthly pursuits and pleasures to benefit from the serenity that he insisted was available to us. Most of us oscillate between being slightly hyper/anxious on the one end and slightly lethargic/melancholic on the other; the Buddha's meditative techniques guide us towards spending more time in between these two extremes, enjoying the equanimity that doesn't come naturally without the deep self-monitoring that he promoted. The freedom to choose our states of mind, rather than being captive to them, was Gautama's most powerful and practical teaching.