People of the same trade seldom meet together, even for merriment and diversion, but the conversation ends in a conspiracy against the public – Adam Smith

As usual, Adam Smith was right. Today I can think of no trade about which the above is more true than the medical profession. I don’t just mean doctors’ use of occupational licensure laws to keep their prices artificially inflated. Politically active groups of doctors are possibly the greatest single threat to personal freedom that there is in the UK today. Their motivation isn't necessarily their wallets, but their egos. Bullies like to use the state to push people around so they feel powerful.

There’s a sad example of this in today’s call in the Lancet, a medical journal that is often used as a political mouthpiece by campaigning doctors, for the government to introduce a “fat tax” to curb obesity.

Of course, the proposal is utterly specious. It's pretty dubious whether the "obesity epidemic" claims are true or not. And which diet plan should be implemented? Is it bacon, sugar, bread or something else that makes us fat? Will political parties of this fat tax utopian future be divided between the Low-Carb Party and the Low-Fat Party? And what if fat people's early mortality rates mean that they actually save the government money in pension and care home bills?

The doctors err even by their own logic. As Will Wilkinson has pointed out, if fat taxers thought things through, they would favour a tax on fat people themselves, not on the food they eat. Taxing food punishes people who exercise so that they can enjoy Big Macs, but not people who are so lazy that they balloon out while eating a balanced diet.

The justification for pushing people around like this is the NHS. Shouldn’t people have to pay for their own illnesses? Well, yes – that’s how personal responsibility works. But having an NHS removes the personal responsibility, and artificial attempts to inject it into the system are doubly illiberal and wrong.

The government (and the electorate, for that matter) forces people to be in the NHS. You have no choice in the matter, and you can’t opt out of it. Jamie Whyte put it well: "first the do-gooders conjure up the external costs by insisting that no one should have to pay for his own medical care, then they tell us that they must interfere with behavior that damages our health because it imposes costs on others." This is perverse and illiberal. The tax would only affect the poor – rich people's spending habits wouldn't be dented. How easy it must be for doctors to pontificate about the need for a fat tax, knowing that such a tax would hardly affect them at all.

This creepy, controlling paternalism has plenty of fans in politics on both sides of the partisan divide. Doctors are the politicians' enablers, lending the weight of their “expertise” to the nanny instinct of the political class in exchange for the feeling of being important. No amount of expertise – medical or otherwise – should give somebody the right to interfere with another adult’s choices. Nor should democracy be used as an excuse to violate the sovereignty of the individual. If fat people are costing the NHS money, that's a mark against having an NHS, not against having fat people.