Emotional AppealType: Red Herring
An appeal to emotion is a type of argument which attempts to arouse the emotions of its audience in order to gain acceptance of its conclusion. Despite the example of Mr. Spock from the original Star Trek television series, emotion is not always out of place in logical thinking. However, there is no doubt that strong emotions can subvert rational thought, and playing upon emotions in an argument is often fallacious.
When are appeals to emotion appropriate, and when are they fallacious? No student would attempt to prove a mathematical theorem by playing upon the teacher's sympathy for the long hours of hard work put into it. Such an appeal would be obviously irrelevant, since either the proof is correct or it is flawed, despite the student's best efforts. In contrast, if the teacher attempts to motivate the student to work on proving the theorem by invoking the specter of a failing grade, this appeal to fear is not irrelevant.
So, one distinction between relevant and fallacious appeals to emotion is based on the distinction between arguments which aim to motivate us to action, and those which are intended to convince us to believe something. Appeals to emotion are always fallacious when intended to influence our beliefs, but they are sometimes reasonable when they aim to motivate us to act. The fact that we desire something to be true gives not the slightest reason to believe it, and the fact that we fear something being true is no reason to think it false; but the desire for something is often a good reason to pursue it, and fear of something else a good reason to flee.
Even when appeals to emotion aim at motivating us, there is still a way that they may fail to be rational, namely, when what we are being persuaded to do has insufficient connection with what is arousing our emotion. For instance, a familiar type of emotional appeal is the appeal to pity or sympathy, which is used by many charities. Photographs of crippled or hungry children are shown in order to arouse one's desire to help them, with the charity trying to motivate you to write a check. However, there may be little or no connection between your check and the poor children you wish to help. Certainly, your money will probably not help the specific children you see in such appeals. At best, it may go to help some similar children who need help. At worst, it may go into further fundraising efforts, and into the pockets of the people who work for the charity.
In such cases, what is needed is an argument that there is a causal connection between the action motivated by emotion and the circumstances that arouse that emotion. Will writing a check help the pitiful children? Will voting for this candidate help prevent frightening circumstances? If all that a charity or candidate does is arouse emotions, that is no reason to give them money or votes. When we feel strong emotions, we want to do something, but we need good reasons to believe that the something we do will be effective.
Douglas Walton, The Place of Emotion in Argument (Penn State, 1992).
Acknowledgment: Thanks to David Short for correcting my Latin spelling.