It's not often that a presidential administration admits it doesn't have the authority to do something it's doing. But that's what the Obama administration's immigration policy boils down to.

"This memorandum confers no substantive right, immigration status or pathway to citizenship," a DHS memo issued last week says. "Only the Congress, acting through its legislative authority, can confer these rights." Yet the same memo gives detailed instructions to federal officials about how they "should enforce the Nation's immigration laws against certain young people."

Secretary Janet Napolitano instructs her employees to use "prosecutorial discretion." But what she actually intends to do is take away discretion by laying out very specific criteria. DHS won't prosecute unlawful immigrants who came to the U.S. "under the age of sixteen," who have "continuously resided in the United States for at least five years," who are "not above the age of thirty," and so forth.

All law enforcement officers have some discretion, of course. A police officer who pulls you over for speeding may opt to write a ticket, or simply issue a warning. There are always exceptions to the rule. But if, for example, the police chief instructs his officers to ignore speeding by anyone under 16, the chief would be taking discretion away and effectively changing the meaning of the law.

This isn't the first time that President Barack Obama has dreamed of overriding Congress and passing his own policies. "The idea of doing things on my own is very tempting, I promise you, not just on immigration reform. But that's not how our system works," he explained to Hispanic activists last summer. "That's not how our democracy functions."

The immediate goal is to undercut Sen. Marco Rubio, a Republican of Florida, who's been working to find a legislative solution to the situation of children who at a very young age were brought by their parents to the United States illegally. As a policy matter, there are reasonable and targeted ways to do this that would uphold the principles of immigration reform and not become a blanket amnesty.

The fundamental problem is that the administration is trying to implement laws that Congress hasn't passed. The Obama administration has overstepped its bounds before, on healthcare, education, the environment, and now immigration. It should rescind the DHS memo and allow the legislative process to provide a real solution to a difficult problem.