Fallacies of reasoning fall come in two kinds. Logic textbooks focus on formal fallacies, slips in inference: “If all dogs have tails, then all animals with tails are dogs.” So-called critical thinking instruction tends to focus on informal fallacies, tricks of distraction: “Scientific people don’t believe in ghosts, so there must not be any ghosts.” “If we don’t change our mind about global warming, we’ll all drown!” “Everyone knows the moon is made of cheese.” "How can you be so unpatriotic as to criticize the President's proposal?"
There certainly are such things as informal fallacies, and in the last four examples, the reasoning really is fallacious. Unfortunately, having been raised on bad critical thinking textbooks, most of my students think all appeals to such things as authority, fear, popular opinion, and shame are illegitimate.
That’s utterly false. Let us think a little more critically about so-called critical thinking.
Take authority: There is nothing wrong with asking a geologist about the chemical composition of limestone, since I can't possibly have first-hand knowledge of everything, and he knows more about limestone than I do. Careful use of authority serves the ends of reason provided that I have reasonable assurance of the supposed authority's honesty, reliability, and qualifications, the question asked concerns his own field of expertise, I consider not just his answers but the reasons he gives for them, and, if authorities differ, I consult the other ones too.
Or take fear: Of course the mere fact that laws are backed up with fear of punishment does not prove that their principles are true. But laws are not illegitimate just because they depend on fear. Law is a teacher, and those who will not listen to reason must be instructed by penalties.
Or popular opinion: We have no inside knowledge about what the moon is made of, but we do have inside knowledge of many other things – for example, the structure of human motivation. So, the fact that almost all people in all times and places would agree that they undertake each of their acts for the sake of some good is a strong argument that they really do. Though we may have forgotten, the classical philosophers understood this well
Even shame: If I really have done wrong, then those who arouse my conscience are doing me a favor. They are asking me to pay attention to the data; they are calling my attention to something that deep down, I already know. Such an appeal to shame is entirely legitimate.