Another topic that bothers me are people who believe stuff they get told by someone without actually questioning arguments in general, or especially their credibility.
For example, alternative medicine. The term itself is debated (for instance, Richard Dawkins said that "there is no alternative medicine. There is only medicine that works and medicine that doesn't work."), and I can't judge every method that isn't already acknowledged as convential medicine, but I would like to state my own experience with a particular "medicine" called Homeopathy, which was first developed by German physician Samuel Hahnemann in 1796.
For those not familiar with it, I quote Wikipedia:
Homeopathy is a form of alternative medicine in which practitioners treat patients using highly diluted preparations that are believed to cause healthy people to exhibit symptoms that are similar to those exhibited by the patient.
It is based on the principle of "potency", which is basically the dilution of the original substance. A 6C dilution, for instance, is the original material diluted by a factor of 100−6=10−12 (one part in one trillion or 1/1,000,000,000,000), what isn't the limit (200C also exists).
Another example given by a critic of homeopathy states that a 12C solution is equivalent to a "pinch of salt in both the North and South Atlantic Oceans", which is approximately correct.
Homeopathy depends on subject word of mouth instead of scientific testing ("The collective weight of scientific evidence has found homeopathy to be no more effective than a placebo.", Wikipedia).
An example for that would be the opinion of a close relative of mine, who firmly believes in Homeopathy.
If confronted by studies and scientific research, all stating that Homeopathy is completely without effect except maybe the placebo effect, the response usually is: Person X told me that Person Y had a headache, took some [insert Homeopathic substance] and got better in no time!
How should one argue with that? Any suggestions?
For anyone interested in this or similar topics, I recommend "The Enemies of Reason", written and presented by evolutionary biologist Richard Dawkins.