I want to talk about bad ideas and good ideas.

Bad ideas originate from many directions. They can be based on the convictions of so-called gurus – the L. Ron Hubbards, or the Andrew Wakefields of this world – whose insane teachings are cherished like nuggets of gold by their many advocates. They can be based merely on a distrust of officialdom, such as is evident in the comments of the New World Order zealots, or the many and varied conspiracy-theorists in our midst. They can arrive from wishful thinking, like belief in angels or the Loch Ness Monster, or the idea that ancient aliens founded cities on the planet long before we arrived. They can be based on literal interpretations of ancient scriptures, evident in fundamentalist interpretations of Islam and Christianity. They can capitalise on fear or feed ancient prejudices, leading to pogroms, slavery and racism.

Bad ideas are like viruses. They are most successful when they exploit the parts of our brain that deal with our strongest emotions – love, fear, joy, loss and hatred. In this way they can persist for generations. Superstitions, astrology, homeopathy, fairy belief, white power, anti-semitism and witch-hunting all have a long, inglorious provenance, but this alone doesn’t make them good ideas. Not one bit.

Bad ideas inhabit a twilight zone, bolstered up by groupthink, forgiven with generous excuses and defended by Byzantine forms of apologetics. When the emperor has no clothes on, attacking the small child becomes the order of the day.

Bad ideas hurt. They sometimes kill. Quack medical practitioners, their heads stuffed with bad ideas, can give advice that endanger their clients’ health. Unscrupulous charlatans can empty the bank accounts of the unwary as they offer them false hope about themselves and loved ones. Governments have gone to war based on bad ideas. Bad ideas cause world leaders to bluster and prevaricate while the world’s climate changes, decade by decade.

Good ideas, by contrast, originate from systems that expose ideas to reality. When ideas don’t work, they are jettisoned in favour of better ideas. Over time, the best ideas rise to the top. Practical trades, such as plumbing and bricklaying, have no time for bad ideas, because they simply do not work. The currency of these professions are good ideas – ones that have stood the test of time, that do what they are intended to do.

Good ideas emerge from science and engineering all the time. We put men on the moon due to a string of great, practical ideas. The computer on your lap, that phone in your pocket, that car you drive, the pacemaker keeping your father’s heart ticking – they all happened because people built good ideas upon good ideas upon good ideas – a solid pyramid of innovation.

Good ideas are hard to come by. Bad ideas are ten-a-penny. In medicine, bad ideas cost lives, so there is a continual search for ideas that have the potential to do great good – to extend the quality of our lives and ease suffering. We’re still not there but each year a few new useful ideas are discovered. In the end, that’s a positive, hopeful story.

We look at race relations differently. We look at human rights and animal rights differently. We look at gender relations and sexuality differently – not because they are the faddish thing to do, but because they concur with objective reality. They match with how things really are when they are put to the test.

I understand the danger of bad ideas. I greatly value good ideas. And that is why I am a sceptic.