The Karma philosophy, in its essence, says that what you do, why you do things and how you deal with what happens to you combine to determine the sort of life you will lead. Prarabda Karma – the Karma of previous births – is considered the reason why some are born poor, some are born rich, some are born healthy and some are born sick.
Karma does not restrict itself to the sort of cause and effect that material science prescribes. It goes into the motives of why you do things as well. When you do virtuous deeds you accumulate ‘Punya’ (Good Karma?) and when you are vicious you accumulate ‘Paap’ (Bad Karma?). Virtue is a combination of deed and motive. Giving to the poor in the spirit of giving is not the same as giving to the poor for the sake of publicity.
Thus, in essence, Prarabda Karma decides what is predestined for you. In effect, then, if your exercise of free will in determining your motives and consequences and your efforts may not much alter your circumstances in this life but they shall certainly determine the sort of life you lead the next time you are born.
Going by the philosophy of Karma, a person who sits on his hands doing nothing since everything is predestined for him is like a person who spends what he saved yesterday and saves nothing for tomorrow.
If you truly believe in a chain of rebirths, then the Karma philosophy gives you the perfect reason why you should work. After all, not everything that you do today gives you results today. Thus, put in the efforts in this birth in order to reap the fruits in the next!