Here we go again. When oral arguments resume in January, the Supreme Court will hear two cases that are likely to send mere laymen climbing the walls in search of common sense. As you may have guessed, the cases deal with the exclusionary rule.

One case involves Charles Steven Acevedo, who was nabbed with a quantity of marijuana in his lunch bag. The other involves a juvenile offender, Hodari D., who took to his heels when he saw a police car. In both cases the question is, Should the evidence thus obtained have been excluded at trial?In recent years the Supreme Court has gone through repeated agonies of indecision over the exclusionary rule. Determining a "reasonable" search or seizure is tougher than pinning down obscenity or defining an establishment of religion. The justices keep trying to find "bright lines," but the only lines they find are smudged.

Take the Acevedo case, to be argued on Jan. 8. It goes back to October 1987, when federal agents in Hawaii intercepted an express package destined for an apartment on Stevens Avenue in Santa Ana., Calif. The package contained nine clear bags of marijuana. Agents let the shipment continue, but kept the address under surveillance.

Shortly after the package arrived, Acevedo entered the apartment, empty-handed, only to emerge 10 minutes later carrying a brown lunch bag that appeared to be full. Acevedo put the bag in the trunk of his car and drove away. Police stopped him, opened the trunk, found the sack, opened the sack, and found the marijuana. Oops! The police, it appears, could open the trunk without a warrant but they could not open the sack without a warrant. The evidence was excluded.

Since at least 1977, the high court has been making such fine distinctions under the "automobile exception" to the Fourth Amendment. Justice Harry Blackmun wearily described the court's several opinions as "vacillations," a charitable term. Cases have turned upon the character of a footlocker and the opacity of a green plastic bag. Was contraband on the seat? In the glove compartment? In the spare tire? Now we shall learn about marijuana in a lunch bag.

The case of young Hodari D., scheduled for argument on Jan. 14, turns upon the meaning of "seizure" under the Fourth Amendment. In 1988 police in Oakland, on routine patrol through a drug-infested neighborhood, saw several boys clustered around a car. As the squad car approached, the driver sped off. Hodari and the other youths scattered. Police officer Jerry Pertoso gave chase. He caught up with Hodari. When they were about 11 feet apart, the boy threw away an object. Pertoso picked it up: cocaine.

The juvenile court judge admitted the evidence and gave the youngster five years of renewed custody as a ward of the state. On appeal, the California Court of Appeal reversed. The court's curious reasoning was that an officer who chases a suspect has effected a "detention." The officer may not have physical control over the suspect, but the suspect's freedom has been infringed by the pursuit. If a person sees a policeman and immediately flees, that is not sufficient cause for the cop to chase after him.

As precedent for this unmitigated nonsense, the California court mistakenly looked to a Michigan case decided by the Supreme Court in 1988. The case involved Michael Mose Chesternut, who was walking along a street when a squad car came by. Chesternut took off. Police kept up with him. Chesternut discarded a packet. It turned out to be cocaine. A subsequent search turned up heroin as well.

The Supreme Court ruled unanimously that "in this particular case" the police conduct did not amount to a seizure, but the court ducked the larger question: Is the act of fleeing, by itself, sufficient to constitute a reasonable suspicion that would justify pursuit? The question that was left unanswered in Chesternut will now be tackled in Hodari.

But tackles can be missed, and tackles can be evaded, and the nine justices will be shaking their heads over this one. The whole purpose of the exclusionary rule - and it is an excellent purpose - is to deter misconduct by police. If the cops cannot introduce evidence that is unconstitutionally acquired, what is the point in acquiring it?

But if the cops are hamstrung in their difficult and dangerous work by weird judicial constructions, the people must be the losers in their war against criminals. That makes no sense to laymen. It ought not to make sense to jurists either.