Karma is a causal concept in Indian religions. The word is Sanskrit for “action” or “deed”, but is typically used to refer to the cycle of cause and effect set in motion by every deed. In particular, the principle of karmic return holds that every act done by a person will return to that person eventually: a good act will be returned with good; an evil act with evil. In Buddhist traditions, this cycle is treated as a natural law of cause and effect, whereas in theistic Hindu traditions, God is responsible for ensuring karmic rewards and punishments.
Since bad things often happen to good people, and vice versa, it is important that the return on good and evil actions is not direct, but can happen much later. For this reason, belief in karma is linked to belief in reincarnation: when an apparently good person endures a life of misfortune, it is the deserved result of evils in a past life. Similarly, when a person apparently gets away with many evil actions throughout their life, karma ensures that this will return to cause them misery in a future life. (In some traditions, the good and evil is returned through reincarnation itself: a person who has done many good deeds may return in a higher caste; an evildoer in a lower caste, or even as a lesser creature.)
Modern science treats cause and effect in terms of mathematical laws which leave no place for the moralistic causality of karma. Furthermore, there is no plausible evidence for the existence of reincarnation (see separate article), without which the principle that good deeds are always rewarded and evil always punished is obviously false – experience provides many counterexamples.
When combined with the theory of reincarnation, the principle of karmic return becomes an entirely untestable supposition. Since we can never know the life history of the entire sequence of past and future selves of an individual, it is impossible to evaluate whether or not the good deeds they have done have been met with equally good consequences. Karma becomes an unfalsifiable principle as soon as it is admitted that the balance of good and evil need not be restored in a single lifetime.
Within most interpretations of the concept of karma, it’s untestability means it is impossible to refute – or support – through evidence.