Logos or the appeal to reason relies on logic or reason. Logos often depends on the use of inductive or deductive reasoning.
Inductive reasoning takes a specific representative case or facts and then draws generalizations or conclusions from them. Inductive reasoning must be based on a sufficient amount of reliable evidence. In other words, the facts you draw on must fairly represent the larger situation or population. Example:
In this example the specific case of fair trade agreements with coffee producers is being used as the starting point for the claim. Because these agreements have worked the author concludes that it could work for other farmers as well.
Deductive reasoning begins with a generalization and then applies it to a specific case. The generalization you start with must have been based on a sufficient amount of reliable evidence.Example:
In this example the author starts with a large claim, that genetically modified seeds have been problematic everywhere, and from this draws the more localized or specific conclusion that Mexico will be affected in the same way.
Avoid Logical Fallacies
These are some common errors in reasoning that will undermine the logic of your argument. Also, watch out for these slips in other people's arguments.
Slippery slope: This is a conclusion based on the premise that if A happens, then eventually through a series of small steps, through B, C,..., X, Y, Z will happen, too, basically equating A and Z. So, if we don't want Z to occur A must not be allowed to occur either. Example:
In this example the author is equating banning Hummers with banning all cars, which is not the same thing.
Hasty Generalization: This is a conclusion based on insufficient or biased evidence. In other words, you are rushing to a conclusion before you have all the relevant facts. Example:
In this example the author is basing their evaluation of the entire course on only one class, and on the first day which is notoriously boring and full of housekeeping tasks for most courses. To make a fair and reasonable evaluation the author must attend several classes, and possibly even examine the textbook, talk to the professor, or talk to others who have previously finished the course in order to have sufficient evidence to base a conclusion on.
Post hoc ergo propter hoc: This is a conclusion that assumes that if 'A' occurred after 'B' then 'B' must have caused 'A.' Example:
In this example the author assumes that if one event chronologically follows another the first event must have caused the second. But the illness could have been caused by the burrito the night before, a flu bug that had been working on the body for days, or a chemical spill across campus. There is no reason, without more evidence, to assume the water caused the person to be sick.
Genetic Fallacy: A conclusion is based on an argument that the origins of a person, idea, institute, or theory determine its character, nature, or worth. Example:
In this example the author is equating the character of a car with the character of the people who built the car.
Begging the Claim: The conclusion that the writer should prove is validated within the claim. Example:
Arguing that coal pollutes the earth and thus should be banned would be logical. But the very conclusion that should be proved, that coal causes enough pollution to warrant banning its use, is already assumed in the claim by referring to it as "filthy and polluting."
Circular Argument: This restates the argument rather than actually proving it. Example:
In this example the conclusion that Bush is a "good communicator" and the evidence used to prove it "he speaks effectively" are basically the same idea. Specific evidence such as using everyday language, breaking down complex problems, or illustrating his points with humorous stories would be needed to prove either half of the sentence.
Either/or: This is a conclusion that oversimplifies the argument by reducing it to only two sides or choices. Example:
In this example where two choices are presented as the only options, yet the author ignores a range of choices in between such as developing cleaner technology, car sharing systems for necessities and emergencies, or better community planning to discourage daily driving.
Ad hominem: This is an attack on the character of a person rather than their opinions or arguments. Example:
In this example the author doesn't even name particular strategies Green Peace has suggested, much less evaluate those strategies on their merits. Instead, the author attacks the characters of the individuals in the group.
Ad populum: This is an emotional appeal that speaks to positive (such as patriotism, religion, democracy) or negative (such as terrorism or fascism) concepts rather than the real issue at hand. Example:
In this example the author equates being a "true American," a concept that people want to be associated with, particularly in a time of war, with allowing people to buy any vehicle they want even though there is no inherent connection between the two.
Red Herring: This is a diversionary tactic that avoids the key issues, often by avoiding opposing arguments rather than addressing them. Example:
In this example the author switches the discussion away from the safety of the food and talks instead about an economic issue, the livelihood of those catching fish. While one issue may effect the other, it does not mean we should ignore possible safety issues because of possible economic consequences to a few individuals.