What is the direct philosophical basis of global justice, as per the passage?

Global justice theories have the manifest goal of universalising the postulates that conceptualise the world as one political entity. Darrel Moellendorf has reasoned that if justice is owed within the borders of a nation-state, the same reasoning supports justice beyond those borders. The philosopher Peter Singer set out his main arguments about the global aspects of morality in his influential essay 'Famine, Affluence and Morality'. He held that suffering and death from starvation, and the lack of basic health facilities or shelter, are simply bad; and that those who have the capacity to prevent something bad without sacrificing anything of comparable moral importance are morally obliged to do so. For Singer the needs of strangers, that is, people other than our own fellow neighbours, are equally morally compelling. In a subsequent book entitled 'One World', Singer argued further that the world is interrelated and intertwined, so we need a transglobal understanding of justice. Thomas Nagel, adding to the debate, says that the least controversial claim one could make in international political theory is that we do not live in a just world. There may be room for disagreement about the most effective methods to deal with problems of global poverty, malnutrition and the death due to associated reasons; but some form of humane assistance to those in dire need is clearly called for, apart from any demand of justice.