Eloquently and powerfully stated:

In addition to being impractical, this principle differs from the common sense morality that we in fact adhere to. We acknowledge an obligation to help the needy, but that obligation is unconditional only in certain circumstances: with family-members, people we have previously agreed to help, or certain kinds of immediate need that appear in our presence””such as the child drowning in a puddle as we’re passing by. If we had more general obligations to aid strangers that were absolutely unconditional””if we HAD to give our money to the street-person asking for it once we confirmed that he needed it to gain something he had a right to””our own negative rights to choose what to do with what is ours would be nullified; a conclusion most of us could not accept. Because there cannot be unconditional duties to provide goods and services, there cannot be unconditional rights to them, however necessary they may be. Our common morality posits an unconditional duty not to harm; but a conditional and limited duty to help.

(via GruntDoc)