Amartya Sen's idea's are of course much too complex to adequately summarize here. To put those ideas in an extremely small nutshell, we can say that Sen believes that economics does not pay enough attention to ethical considerations. The field essentially holds that ethics are separate from proper "positive" economics.
Economists hold that people always act from self-interest. They hold that people think about how to achieve their self-interest in a very simple way. That is, we are said to decide what to do by weighing its cost for us (marginal cost) against its benefit to us (marginal benefit). We choose the actions that get us the most benefit at the lowest cost.
Sen argues that this is not true. He argues that people have much more complex motivations than this. He argues that people care about how others perceive them and they care about social norms. Sen believes that economics must pay more attention to the sorts of ethical concerns that drive people. It must, as a field, try to think about which goals are best at bringing about social justice.
Thanks for your explanation. You have clearly explained the need for ethics in economic world and the importance of social justice. But according to A. Sen, how to realize that? does he propose any model or theory to bring ethics and economics togather? Why should the self-interested model of economics be depromoted? If you could enlighten me with some answers, i would be really happy. thanks in advance.