It occurred to me recently that the cycle of rebirth, samsara, is essential to yoga. In fact, if samsara was not real, there would be no point to yoga. Yoga's stated goal is the stilling of the fluctuations of the mind, but really this is done in order to perceive one's purusa, or Isvara. And when one perceives one's purusa, or one's atman, or Isvara, one finally identifies with one's true self, and leaves the realm of prakrti and the cycle of samsara.
Tied up with the cycle of samsara is the law of karma. To simplify it slightly, it's essentially a law of cosmic justice. You sow seeds of karma, and
you personally must reap the consequences of those seeds, be they good seeds or bad ones. You continue to do this over many lifetimes as long as you continue to sow seeds.
Both of these concepts, samsara and karma, are essential to the yoga practice, because without them yoga has no purpose, no need to exist. You don't need a solution for a problem that doesn't exist. This is why I think it is necessary to ask whether or not these two concepts are in fact true. Personally, I'm doubtful of both, but this may only be because of my modern way of thinking. I'm more inclined to favor a psychological explanation* of these two concepts than to
actually believe them.
But at the same time, I'm drawn to the practice of yoga. I find many principles and practices that I not only agree with, but of which I have actually experienced the benefits. Perhaps as I proceed through yoga, an understanding of the more complex concepts will become clearer, but for now I remain a half-convinced, half-suspicious practitioner of yoga.
*Very loosely paraphrasing Nietzsche, concepts like Karma (or what I called cosmic justice) are merely inventions by people who are trampled upon by more powerful people. The trampled people shake their fists and say "You just wait! You'll get what's coming to ya!" while the powerful people conquer nations, take hundreds of wives, and shower in gold. Karma starts as an invention to help trampled people feel better, and ends up as a way of psychologically suppressing powerful people.