Singer believes that a kind of universalization is built into our moral concepts. If we say that it is wrong for person A to do X, it follows logically that it is wrong for any similar person to do X in similar circumstances. He calls this the generalization principle.

He also believes that if the consequences of per A doing X are undesirable, A ought not to do X. He calls this the principle of consequences. 

He combines these two principles into an argument he calls the generalization argument. The argument concludes that if the consequences of everyone doing X are undesirable, then no one ought to do X without a justifying reason to do so.