Arguing Immigration with a Compassionate Liberal -or- How to Twist Your Head into a Pretzel

The following article by David Haggith was published first on The Great Recession Blog:
Arguing with a liberal about the economic impact of rampant immigration will twist your brain into a pretzel. It inevitably goes something like this:
“Illegal aliens and legal immigrants are taking millions of our jobs.”
“No, they’re not.”
“Then they’re all on welfare.”
“No, they’re not. Even most undocumented workers are productive members of society.”
“How are they productive if they’re not taking jobs?”