IHATE talking about "slippery slopes."

It's such a cliché, as trite as "Pandora's box" or "a can of worms." It just sounds classier.

But there are times when the only thing that fits, the only phrase that accurately describes the consequences of an ill-thought-out action is that greasy little hill. And given what Seth Williams is planning to do with some drug offenses, it seems that we'll be sliding away on a sea of total grooviness really soon.

I'm talking about the D.A.'s decision to downgrade possession of 30 grams or less of marijuana from a misdemeanor to a summary offense. Now, that might not mean anything to those of you who, like me, never smoked a doobie. But it means a lot for the criminal-justice system, particularly police officers whose job it is to arrest people who are - and let's be clear on this - breaking the law.

THANKS TO Williams and his judicial mentors Seamus McCaffery and Ron Castille, a person apprehended with less than 30 grams of Mary Jane cannot now be charged with a more serious misdemeanor offense, which carries possible prison time and gives them a record. Starting at the end of this month, these minor-league dopers will have to pay a fine (maybe) but face no risk of a true prosecution. And their records remain as clear as an unsmoke-filled room.

How nice. How tolerant. How reassuring to the rest of us who actually respect the federal Controlled Substances Act and its state counterpart.

Williams was quoted in an Inquirer article as saying, "We have to be smart on crime. . . we can't declare a war on drugs by going after the kid who's smoking a joint on 55th Street. We have to go after the large traffickers."

Well, the reason that the large traffickers are in business, counselor, is in part because that kid on 55th Street is part of a willing market.

The fact that you only catch him smoking weed on Monday doesn't mean he doesn't have a stash at home for the six other days of the week.

And the implication that a guy who's breaking the law shouldn't have a criminal record is appalling, especially when our city's top law-enforcement officer is making it.

I don't care how common it is for people to kick back with their happy sticks and get mellow occasionally, it's up to the Legislature, not the D.A., to determine whether a violation of the drug laws should be dealt with more seriously than, say, allowing your puppy to poop on the stoop.

And then you have the argument trotted out by people who always seem to have an eye on the inequities of the justice system that prosecution of drug offenses is unbalanced and unfair. They always like to point out that it's much more likely for a kid from 55th Street to end up in jail than his counterpart in Lower Merion.

I totally agree. But the response is not to make it easier for the kid on 55th to skate with a slap on the wrist. It's to maximize the probability that his friend in Lower Merion is going to be looking at some vertical bars, or at least have to deal with the shame of a rap sheet.

Speaking of which, a study issued by the Office of National Drug Control Policy last year showed that at least half of the men arrested in 10 major cities for other crimes tested positive for some type of illegal drug. The big winner? Pot.

What was that about there being no connection between marijuana and crime?

The fact that law-and-order types like Castille and McCaffery signed on for this deal is troubling, especially since it gives cover to the Philly D.A. Hard to criticize Williams when they've got his back.

But perhaps they've been breathing the rarefied air of the courts for too long. They need to get back to ground zero, the streets, where it's a quality-of-life issue.

The policy shift may seem insignificant since Williams took pains to emphasize "we are not decriminalizing marijuana."

Not yet, maybe. But the fact that Philadelphia no longer believes possession of enough marijuana for 60 joints is no longer a grown-up crime is troubling.

Fortunately, the police aren't buying it. As Lt. Frank Vanore stated, "We're not going to stop locking people up." But unless they get cooperation from the D.A., their efforts to uphold the law will go up in smoke.

Christine M. Flowers is a lawyer. Listen to her Thursdays on WPHT/1210 AM, 10-midnight.