The relationship between politics and science has never been easy, but there has rarely been a more embarrassing mismatch than in our drug laws. Supposedly a measure to protect the health of the nation, we have arrived at a situation where some of the most dangerous drugs are legal, some of the least dangerous are prohibited, and where many of the dangers from drug use arise from their illicit supply. But even by the standards of this self-imposed prohibition of science, the new Psychoactive Substances bill is a work of monumental ignorance that has taken drug legislation beyond the point of farce into the realm of surreal fantasy.

The motivation behind the bill is the wave of new psychoactive substances or legal highs. Grey market labs have rifled the scientific literature to create substances that produce similar effects to popular street drugs like cannabis, ketamine and ecstasy, but are different enough to avoid existing bans and are often significantly worse for your health. To try to address this problem, the government is trying a radically new approach: pretending that one of the most difficult problems in neuroscience – and one of the deep mysteries of consciousness – doesn’t apply to them. It’s a bold move, to say the least.

The bottom line is, the only way of knowing whether a mystery substance alters the mind is to take it

Rather than banning a specific list of drugs, the government wants to outlaw the supply and production of all psychoactive substances and have a minimal list of government-approved highs. Unsurprisingly, booze, nicotine, and caffeine are allowed, alongside, rather vaguely, “any substance which is ordinarily consumed as food” but isn’t already banned.

But the scientific K-hole here is the fact that the law relies on adequately defining a “psychoactive substance”, which turns out to be scientifically impossible at the current time. It’s not that you can’t come up with a definition; in fact, the bill says it’s something that “by stimulating or depressing the person’s central nervous system, affects the person’s mental functioning or emotional state”. The problem is turning this into a law that unambiguously classifies substances as psychoactive or not.

The bottom line is, the only way of knowing whether a mystery substance alters the mind is to take it. You simply can’t tell by chemical tests, because there is no direct mapping between molecular structure and mental experience. If you could solve the problem of working out whether a substance would affect the conscious mind purely from its chemistry, you would have done Nobel prize winning work on the the problem of consciousness. A second-rank approach is just to see whether a new substance is similar to a known family of mind-altering drugs, but even here there are no guarantees. A slight tweak can make a similar drug completely inactive and about as much fun as Theresa May at a techno night.

Facebook Twitter Pinterest Just said no: Theresa May rejected scientific recommendations made by the Advisory Council on the Misuse of Drugs. Photograph: Lauren Hurley/PA

This is exactly the same problem that pharmaceutical companies face when developing psychiatric drugs, by the way. They can analyse molecules and give them to mice, but the true test – the acid test, if you will – only comes when a human swallows it. Labs that produce new legal highs use the simple expedient of giving them to their mates to test. But this liberty isn’t available to courts because “have a blast on this, your honour” turns out not to be a valid legal argument and giving mystery chemicals picked up by the police to human guinea pigs is a step too far even for the Home Office.

This exact scientific problem is why Ireland’s 2010 Psychoactive Substances Act, on which our new bill is based, had reportedly resulted in only four prosecutions by June 2015. Rather embarrassingly for the UK government, the minister in charge of the Irish national drugs strategy, with the full support of the Irish police, recently announced that Ireland will enact a “radical cultural shift” towards decriminalisation for recreational drugs, including cocaine, heroin and ecstasy, making the law on which our new bill is based obsolete.

This reliance on scientific impossibilities is really just a symptom of a wider neglect of an evidence-based drugs policy. You can see it throughout the process. At the end of October, May wrote to the Advisory Council on the Misuse of Drugs about the bill, as she is required to by law. In line with tradition, she rejected their scientific recommendations but she also wrote to assure them that homeopathy, a practice based entirely on pseudoscience involving sugar pills with no active ingredient, would be specifically excluded from any ban. It’s the scientific equivalent of writing to MI6 to guarantee that crystal balls won’t be restricted under new spying legislation. It would be funny if it wasn’t so tragic.

Despite these recent examples, this is not a party political issue. The drug law charade was equally embarrassing under Labour, when the then home secretary, Alan Johnson, fired the head of his own drugs advisory committee for pointing out scientific evidence he didn’t want to hear. Previous governments fared no better.

It would be naive to think that legalisation or decriminalisation would solve all our society’s drug problems, but our best evidence suggests that problem drug-taking is better dealt with as a medical issue. Near-blanket prohibition has failed to address drug harms while mainly profiting the criminal underworld. Our politicians have made a habit out of rejecting science, and we’re left with the comedown.