The kamma the Buddha taught was different to the karma of his day. The Buddha's kamma is focused on intentionality. This intentionality shapes your internal world and chemical makeup, as well as shaping how you interact with the world.
All mentions of rebirth due to kamma are referring to the rebirth as described here. Every action with intent, including conditioned reaction, falls under the umbrella of kamma.
If you kill someone you will be reborn a tiger. All this means is that if you kill someone you will be reborn with a mind more inclined to kill again. This is the type of rebirth he was talking about.
The clearest sutta describing his view on kamma is here.
Conditioned responses also fall under kamma, and can be removed gradually through right concentration, right effort and right mindfulness. An example of a conditioned response is a near-instinctive reaction to people of a certain appearance due to a past negative data. The data can be real, from experience, or implanted, from gossip or media. This is part of how kamma is spread from person to person.
The 4 types of deed the Buddha describes are:
Dark deeds = painful experience
Bright deeds = pleasurable experience
Mixed deeds = mixed experience
Deeds without intent = liberation / cessation of kamma
His words on dark deeds are:
When someone makes hurtful choices by way of body, speech, and mind. Having made these choices, they’re reborn in a hurtful world, where hurtful contacts strike them. Touched by hurtful contacts, they experience hurtful feelings that are exclusively painful—like the beings in hell.
The simplistic and mistaken understanding of karma is that if you do something bad to others then it comes back to bite you. This is the fruit of kamma, and not what the Buddha focuses on.
The Buddha's focus is how these deeds affect our own internal world. The second we even think a dark thought (a dark deed of the mind) we make our own world worse. We start to obsess about how much more someone has than us, how they might do us harm, how we might scheme and steal from them, or they from us, whether our manipulations will work, and we lose the ability to live in peace. We are reborn in a hurtful world, where hurtful contacts strike us, with most of those hurtful contacts being figments of our imagination. Cortisol and adrenaline dominate.
A similar thing is said of bright deeds:
When someone makes pleasing choices by way of body, speech, and mind. Having made these choices, they’re reborn in a pleasing world, where pleasing contacts strike them. Touched by pleasing contacts, they experience pleasing feelings of perfect happiness—like the gods of universal beauty.
This is the feeling of wellbeing you get when you know you have done something to help someone. People may not always be nice, but you will have a far better time of things if you see the truth: they are living in a dark world, and they see hatred and greed wherever they turn.
Good actions will result in your rebirth in a positive realm; a positive brain chemistry. Serotonin and oxytocin, but these are subject to change too; better not get attached.
The third one sounds complicated but isn’t really. Don’t overthink it.
And what are dark and bright deeds with dark and bright results? It’s when someone makes both hurtful and pleasing choices by way of body, speech, and mind. Having made these choices, they are reborn in a world that is both hurtful and pleasing, where hurtful and pleasing contacts strike them.
Let’s say you’re an athlete and sprint to the finish line to overtake someone at the last second, only to see them crying after the race. Or in business you introduce a friend to an employer you think is good, but who turns out to be an abuser. Your fee sits in your account while your friend ends his life. That one was tough.
The fourth is the key to liberation:
What are neither dark nor bright deeds with neither dark nor bright results, which lead to the ending of deeds? It’s the intention to give up dark deeds with dark results, bright deeds with bright results, and both dark and bright deeds with both dark and bright results. These are called neither dark nor bright deeds with neither dark nor bright results, which lead to the ending of deeds.
This is the intention to abandon all deeds driven by craving or aversion, aiming instead for freedom from intentional action and the ending of kamma.
The existing kamma will come to fruition: everything you have said and done to date needs to work its way out of your system in the form of sankhāra or somatic release. You have to go through the dark night. This is where the meditations of concentration and mindfulness really come into play. Through sitting and observing, in the right chemical makeup, you are able to release the tensions that are stored in your body from past deeds.
After that, it's a case of taking good actions but not being attached to their outcomes. The important thing in buddhism is the intention, so you want to ensure that your intention is not rooted in the false sense of self, but rather in the greater good. You also need to avoid becoming attached to positive outcomes because these are also rooted in the fetter of craving.
Kamma is a deed committed with intent to produce a result. The best kind is a good deed with no attachment to the result.
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