"Logic is the anatomy of thought."
– John Locke
"Logos" is the appeal to logic. Logos isn't logic like the formal logic in math, philosophy, or even computer science; it is the consistency and clarity of an argument as well as the logic of evidence and reasons.
In formal logic, in abstraction, the following is the case: if A is true and B is true and A is an instance of B, then the repercussions of B will always be true. The problem, however, is that this kind of logic doesn't work for real-life situations. This is where argument comes into play. Formal logic would say that speeding, for example, is a violation of traffic laws. A repercussion of violating a traffic law is a ticket; therefore, every person who speeds gets a ticket. However, in real life, not in abstract theory, things aren't that cut and dried. Most people would not agree that all speeders, in every circumstance, should receive a ticket. In an argument about a real-life situation, the audience needs particulars to make their decisions. Sometimes there's an exception. Why was that person speeding? Well, if an eighteen-year-old is speeding to show off for his friends, then yes, most people would agree that he deserves a ticket. However, if a man is driving his pregnant wife to the hospital, then maybe he does not deserve the ticket. One could, and probably would, make the argument that he should not get a ticket.
Let's examine how the appeal to logic would work in an argument for the speeding father-to-be.
Because arguments are based on values and beliefs as well as facts and evidence, it is logical that the argument must coincide with accepted values and beliefs. The enthymeme is the foundation of every argument. Enthymemes have three parts: the claim, the reason, and the unstated assumption that is provided by the audience. All three of these things must make sense to your audience in order for your argument to be considered logical. The claim of an argument for the father-to-be could be something like, "This man should not get a speeding ticket." That's it. The claim is pretty simple. It is your educated opinion on the matter. The reason would be something like "because his wife is in labor in the backseat." So the two stated parts of your enthymeme would be, "This man should not get a speeding ticket because his wife is in labor in the backseat." Now, this seems obviously logical to us; however, what is our underlying value, our unstated assumption about this argument? Most of us would probably agree that a hospital is a better place to give birth in than a backseat. That is the third part of the enthymeme. Your audience must agree that your assumption is true in order for your argument to be considered logical. If your readers don't have the same assumption, they are not going to see your logic. You must find an enthymeme that works for your audience. The pregnant wife enthymeme is fairly easy to see. In more volatile claims and reasons, the unstated assumptions can be trickier to identify and work out with your audience.
Reasons like "because his wife is in labor" are motivations for the driver's actions, not evidence. Most audiences need facts. Evidence is the facts. Both reasons and evidence are used in an appeal to logic; however, reasons cannot be your only support. Evidence as to why the man should speed might include studies about the problems with births in difficult or dangerous circumstances, interviews with women who have given birth in automobiles, and infant mortality rates for births that do not occur in hospitals. As you can see, there are many different kinds of evidence you could provide for this argument.
Consistency means not changing the unstated or stated rules governing your argument. Consistency is essential to logic. Let us continue with the speeding example. If, for instance, you are arguing that the infant mortality rate is too high for babies born outside the hospital and that the father is required to speed for the safety of his unborn child, then you may not want to include evidence of the high infant mortality rate in car crashes. Although this information may be part of the infant mortality rate, it goes against the underlying assumption that speeding is acceptable because of the high risk of harming the baby if it is born in the backseat.
So why should you care about logos? In your own writing, logos is important because it appeals to your readers' intellects. It makes you readers feel smart. Logos is the part of the argument where you treat your audience like purely rational, "only the facts, ma'am" kind of people. Also, gaps, leaps, and inconsistencies in logic, no matter how well developed the other appeals may be, can tear apart an argument in short order. This is the same reason you cannot ignore logos in others' arguments either. All the appeals are linked together; for instance, if you use as evidence an article that has leaps in logic, or relies only on authority and emotions, this article could damage your own ethos as an author. It is important to remember that all three appeals must be well developed and work together to make a good argument.
As you now know, logos can be defined as a writer's or speaker's attempt to appeal to the logic or reason of her audience. Let's look at some examples of logos that you might commonly find when reading texts of various media: