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Acharya Jayant has also stated the same thing:
"The diversity of the world, from happiness to sorrow, is due to the difference in karma. Even in agriculture, service, and other activities, there is a unique outcome. Some people get unexpected wealth, while others are struck by lightning. Some achieve success without effort, while others fail even after great effort. These things are not due to any visible cause. Therefore, one should assume that there is some invisible cause.
(Nyayamnjari, page 42, second part)
In other words, some people are happy in the world, while others are unhappy. Even after doing things like farming, working, etc., some people get special benefits, while others have to suffer losses. Some people get sudden wealth, while others are struck by lightning while sitting. Some people achieve success without effort, while others do not get results even after great effort. All these things do not happen due to any visible cause. Therefore, one should assume that there is some invisible cause.
Similarly, theists generally agree on this point:
"Karma is the main thing in the world. Whatever one does, one gets the fruit of it."
The nature of karma
Although there is agreement between theists and atheists, soul believers and non-soul believers about the theory of karma, there is a fundamental difference of opinion regarding the nature of karma and its relationship to its fruits.
In common language, whatever is done is called karma. For example, eating, drinking, walking, talking, thinking, painting, etc. But karma does not only mean this.