The third renunciation thought is karma. Karma means that whatever we do comes back to us. When we love, love is returned to us. When we hate, we receive hate in return. That is the internal system of karma.
The process is the same in the external karmic system. When we plant rice seeds, we get rice. When we plant corn seeds, we get corn. Everything manifests according to its causes and conditions, which create inevitable results. This is not Buddhist dogma; it is a law of nature.
It is important to contemplate this so we know what is good and what is bad for ourselves and others.
Even an action that appears insignificant can have enormous consequences. Please be intelligent regarding karma, and very careful with your actions, because a tiny seed can bear a lot of fruit.
When we are fully aware of the inevitability of karma we empower ourselves. We all want happiness; no one wants suffering. Karma teaches us that positive actions bring positive results, which lead to happiness, and negative actions bring negative results, which lead to suffering.
A combination of positive and negative actions brings results that are a mixture of happiness and suffering.
What is the source of negative and positive actions? The great master Nagarjuna said that actions based on ignorance, attachment, and anger are negative, and actions based on wisdom, nonattachment, and loving-kindness are positive. This means that we create our own karma by what we think, say, and do—we are the agents and creators of our happiness and suffering.
Taking this to heart, we should work continuously to uproot all our negative mental tendencies and to cultivate and glorify the positive ones. We should continually manifest this in our words and actions for ourselves and others. This is the way to properly work with karma.
Source: Sherab, Khenchen. The Nature of Mind (pp. 120-121). Shambhala. Kindle Edition.