it's not really surprising that many of the arguments used by people opposing science and supporting pseudoscience fall into the category of logical fallacies. Some fallacies, however, are used so often that they are almost impossible to avoid in any discussion with true believers. Here are just four of them.
"No True Scotsman"
This is the tactic of denying the relevance of some unfortunate statement, action or event. A recent example in the news was of a Traditional Chinese Medicine practitioner who recommended "slapping" therapy for a child who subsequently died from the beating. There were immediate claims that this was not a form of TCM, although the boundaries of TCM always seem so remote that almost any form of bizarre behaviour or concoction can fit within the genre. A few years ago a naturopath in NSW was charged with manslaughter following the death of a child. The baby had been booked for emergency heart surgery, but the parents were convinced to cancel the surgery and have the baby treated "naturally". While the trial was proceeding there were many cries from the alternative medicine industry of "witchhunt", but as soon as the verdict was handed down he was no longer a true naturopath, despite having run a practice called "Naturopathy Clinic" for more than 20 years.
The use of this fallacy is not confined to the alternative medicine business. I recently criticised a piece of inane nonsense in a book that is part of the canon of feminist intellectual literature. I was immediately told that the author, despite being a high priestess of the movement who is regularly cited as an authority, was not a true feminist and therefore her idiocy could be ignored.
This term essentially means "you first". The classic response from supporters of cancer quacks is that people still die from cancer when treated by real doctors and that therefore criticisms of quackery should be held back until real medicine sorts out its problems. It is one of the fallacies used by climate change deniers who say that Australia doesn't have to do anything to reduce emissions and the consumption of fossil fuels until other countries do it first. An analogy would be the airline industry refusing to address safety until the road toll had been reduced to zero, although I can't imagine even the most ardent denialists flying anywhere if this was the case.
"The Nirvana Fallacy"
This is the idea that unless something is perfect it is useless. It is a staple tactic of the anti-vaccination movement, who point to the fact that vaccinated people still catch diseases as evidence that vaccines don't work at all. Is also a standard of the cancer quackery industry, where, for example, the fact that not all cancers can be cured by chemotherapy proves that chemotherapy is useless in all situations. Again, it is used by climate change deniers, who will argue that any government policy which doesn't result in immediate reduction of greenhouse gas emissions should be delayed until efficacy can be guaranteed.
Strictly speaking this means an attempt to counter an argument by referring to something irrelevant about the speaker, but it has degenerated to the point where the charge is made if someone gets offended by something and it is interpreted as "saying something nasty about the speaker". There is a very big difference between "You are an idiot for saying that" and "You are only saying that because you are an idiot". The second of these is actually ad hominem, whereas the first may be a statement of fact, although impolite and not good debating practice. A problem arises if the discussion then diverts to the definition of ad hominem and the real topic gets ignored.
The outstanding example of ad hominem is what has come to be known as "The Big Pharma Shill Gambit", where the accusation is that an argument or facts can be ignored because the speaker is being paid to say them, although this payment can take the form of grants to an employing university at several arms' lengths. My support for vaccination was first attributed to me being on the payroll of some unnamed organisation in 1999, and someone claimed to have evidence of the benefits I received (free accommodation at luxury resorts). (Never joke - I once launched a tsunami of abuse for saying that Pfizer owed me a new Ferrari because mine was scratched.)
These are just a few of the fallacies that appear regularly on Facebook and other forums. Others are "Straw Man", arguing against a position that the other person doesn't hold ("You say that vaccines are 100% effective"), and "Non-Sequitur" which is diversion to something irrelevant ("How can chemotherapy work? Antibiotics don't cure the flu.") but I'll leave these for another day.
This article was published as the Naked Skeptic column in the July/August 2015 edition of Australasian Science