If someone told you that alchemy must be a legitimate science because Isaac Newton practiced it, would you start trying to turn lead into gold?
Of course not. Yet that is exactly how the argument from authority works – by replacing logic and evidence with the name of a respected or powerful person.
Whenever your hear such a name used to back up an argument, you should immediately ask yourself two questions:
First, is the named person a relevant expert on the topic? Francis Crick is a legitimate authority on genetics. Oprah Winfrey is not. If, however, you are discussing media entrepreneurship, Winfrey's perspective could offer valuable insights.
Second, does the authority's position make sense? Although Isaac Newton had reasons to believe alchemy might be true in his day, the evidence has since led us to abandon transmutation for modern chemistry. It does not makes sense to practice alchemy based on Newton's stance on the matter.
The inherent caveat of any argument from authority is this: no matter how high on the totem pole a person may be, no matter how much expertise on a subject he or she may have, it is always possible to make mistakes. Even the brightest of us is still only human.
Whenever someone flashes a big name to boost an argument, always be suspicious. Names are only as good as the ideas behind them.