Since medical marijuana was legalized in California in 1996, use has been widespread. And once the Obama administration reduced the harassment, the number of dispensaries has grown rapidly. Not that pot was ever that hard to get out West, but it is now fair to say that the "medical" qualification is close to irrelevant.

So marijuana is now de facto legal in California, requiring only a couple hundred bucks and a short doctor's visit to become a qualified purchaser. Perhaps as a result, a ballot initiative to fully legalize marijuana is polling at about even odds in the Golden State, and marijuana initiatives are in the pipeline elsewhere.

Now, any libertarian must raise a cup, pipe, vaporizer (or whatever) to finally seeing a little bit of progress in the demented War On People Who Use Some Kinds Of Drugs. Combined with the resurgence in research on medical uses of psychedelics—which often find positive benefits—it looks like this may be the beginning of a positive shift in America's drug policy. Slow, partial, and late, but in the right direction.

Change, however, often causes backlash, and we need to be prepared with the right arguments to make sure that the right lessons get drawn from this experience. It is far too easy to make superficial slippery slope arguments against medical exemptions—as early as 1997, Reason's Nick Gillespie quoted Clinton drug czar Barry McCaffrey as calling medical use "a stalking horse for legalization," a phrase frequently repeated since.

In private, we will of course celebrate a moral victory. But in public, our best tools are those which do not depend on appeals to moral values that many voters do not share. Hence the importance of framing this as a successful experiment, a perspective which can serve as an antidote to the irrational fear that prohibitionists will attempt to generate with slippery slope arguments.

The classic slippery slope argument says that if we do A we will inevitably go on to do B, and while A would be a good thing, B would not. In this case, suppose we see medical exemptions for pot leading later to legalization in one state and then others. Opponents would then point to this transition as a reason not to pass future medical exemptions for other psychedelics, because even if medical use is a positive, legalization is not. They would then conclude that we should hold a hard line against any legal use of Schedule I drugs. Or as Robert DuPont, the first director of the NIDA argued in a letter to The Washington Post, "Medical marijuana is a stalking-horse for legalization. This can be seen in California, where medical marijuana advocates have had great success and are pushing for full legalization."

Now, many of us would take issue with the claim that legalization is a negative (Milton Friedman, for example). But this should not be our only tactic, and I'd like to suggest a very different approach, born from the philosophy of competitive, experimental government we advocate on Let a Thousand Nations Bloom. This is nothing new—it's the philosophy that America was founded on over two centuries ago, the idea of states serving as experimental laboratories.

From this perspective, we can fight back against the assumption that medical exemptions will always lead to legalization. We can argue instead that medical use serves as a limited experiment—a sort of partial legalization for the cases most likely to have more benefit than harm. Marijuana for cancer patients, for example, or therapists giving Ecstasy to vets with post-traumatic stress disorder.

Nor is this experiment a mere formality, inevitably leading to full legalization. Suppose we lived in the counterfactual world of Reefer Madness, where a few puffs of sweet ganja turned an ordinary citizen to violence, promiscuity, suicide, apathy, or whatever the current worry of the elder generation is. Then the result of medical marijuana would have been a disastrous rise in these outcomes—and a repeal of the experiment. Instead we have seen nothing particularly bad happen. People got high (with less cost and less stress), the sky did not fall, and so pressure is growing to expand the experiment.

Like any experiment, medical use is imperfect. Negative effects might occur years later (teenage pot use increasing mental illness risks). Of course, so might positive ones—like the neural growth promoted by ketamine and LSD. Interpreting these experiments correctly is far from straightforward, but it is through such imperfect experiments that we Enlightenment-era humans accumulate scientific knowledge.

Regardless of the underlying morals driving prohibitionists, most public arguments for banning substances are based on claims about those substances' harmful effects. This is a fundamentally empirical claim, and like any controversial empirical claim, experiments are absolutely crucial in resolving it. Regardless of our politics, surely all sensible people can agree that more accurate beliefs about the world are valuable for shaping optimal policies.

Unfortunately, not all our political opponents are sensible, but it still behooves us to argue for experimentation. The sensible opponents will agree with the need for experiments, but make different predictions about the outcomes. Meanwhile, those who oppose medical exemptions for promising experiments will be revealed for the irrational prohibitionists they are—and their agenda will get less sympathy when we show its anti-scientific nature.

Experiments and the accumulation of knowledge are what drive human progress. Alcohol prohibition was a failed experiment—and we learned from it. Medical marijuana has been an experiment—and we're learning from it, too. Let's keep experimenting, keep learning, and move towards policies based on accurate facts, not irrational bias. After all, the truth is on our side. Unlike our opponents, we can afford to move forward one experimental step at a time.

Patri Friedman, grandson of economist Milton Friedman, is the founder of the Seasteading Institute.