The Supreme Court of Colorado, where marijuana use is now allowed under state law, is considering whether workers can be fired for smoking pot outside of working hours. The case, which the court is expected to rule on sometime in the coming weeks, concerns a prescription cannabis user, but the ruling may change the ground rules for recreational smokers as well.

Back in 1992, when pot legalization still seemed like an unlikely proposition, political philosopher Nicholas J. Caste argued in the Journal of Business Ethics that punishing workers for drug use—whether legal or not—is morally wrong.

Caste takes on a justification for drug testing that rests on keeping workers productive. The logic behind that position is that employers don’t buy finished products from workers, as they would from independent contractors. Instead, they buy their time. And that means that they have the right to make sure employees use the time as efficiently as possible.

Caste notes that this line of reasoning could permit any sort of “manipulation” of a worker. Keeping an employee from taking drugs might be in his or her own best interests, but that doesn’t matter one way or the other to the argument. If productivity can justify keeping workers from using drugs, he writes, it could just as easily permit forcing them to use drugs.

Caste proposes a thought experiment in which an employer requires employees to use one of two productivity-boosting drugs while off the clock: “hedonine,” which has a side-effect of clear-headed euphoria, or “pononine,” which produces pain along with hard work. Even in the case of pononine, he writes, “Since such a drug would affect an employee’s productivity, its use would be permissible by the productivity argument. … The employee is free at any time to resign his or her position and so is not in any way being coerced.”

If that makes you uncomfortable, that’s exactly Caste’s intent. “The fact that the productivity argument supports such conclusions provides good evidence of its invalidity,” he writes. “If the productivity principle provides the moral basis for employee drug testing, then it also provides such a basis for the use of hedonine and pononine.”

The point is, it’s not just the fact that pononine causes pain that makes us cringe. It’s that demanding that workers use a drug represents a clear intrusion into their lives beyond the workplace, without any regard for their individual needs.

“Demanding a standard of behavior outside of the workplace places the individual employee in much the same position as the medieval bondsman,” Caste writes. “As long as he or she wants to work for a particular employer, that employer is by this account thought to have control over any and every aspect of an employee’s life that may possibly affect his or her productivity.”

Whatever the Colorado court rules, the moral questions that Caste raises are worth consideration as companies set and revise their drug testing policies.