Karma or Choice?

Everyone has heard of karma. Hated and blamed at the human level, karma is seen as a kind of executioner who comes to punish us for deeds committed long ago and deeply forgotten, a sort of self-sustaining force that operates according to a set of mysterious and unknown rules. This would be the bad karma. In moments as rare as the appearance of a rainbow, karma becomes merciful and rewards us for something as unknown and vague as the deeds for which we were punished. This would be the good karma.

But what is karma?