After the completion of the Upanishads, the interconnected doctrines of karma and samsara became prominent in Hindu thinking and persist even today. Both have to do with a philosophy of action and the results of that action. According to the ethical concept of karma (often spelled kharma), the actions or karmas of individuals in their current births shape their lives in their next births. This is connected to reincarnation, or the cycles of lives. Souls are believed to cycle through human or animal lives until they are liberated and merge with a higher reality.