Argument is the attempt to prove a point using evidence and reasoning.
This handout discusses evidence. I cover reasoning in a separate handout.
A claim is a statement about something, which could, in theory, be supported with evidence. It is an assertion about the way things are, or were, or will be, or should be. Claims are, almost by definition, controversial, in the sense that not everyone agrees with them. That is why they require evidence.
Evidence is the concrete facts used to support a claim. Ideally, evidence is something everyone agrees on, or something that anyone could, with sufficient training and equipment, verify for themselves.
Evidence comes in many types, which I discuss in detail here.
At its most basic, evidence is something that can be perceived with the senses. A great deal of scientific evidence is observable phenomena, like the change in color when two chemicals are mixed in an experiment, or the light emitted by a distant star, or the structure of bones in a fossil. In some cases, though, what you’re observing is very removed from the claim. For example, if I say “Einstein said that space is curved” as a fact to support my claim, the observable fact is the written record of what Einstein said, not a physical phenomenon that relates directly to my claim.
There is no clear-cut line between a claim and a “fact.” Things that are facts in one context might be claims in another. For example, “the earth orbits the sun” was once a claim (i.e. an unproven or controversial assertion), but now nearly everyone accepts it as a fact. Many arguments begin by treating something as a claim, try to prove it, and then, once they have proved it to the author’s satisfaction, treat it as a fact to be used as evidence to support some new claim. However, in well-constructed arguments you can usually distinguish pretty easily between the claim and the facts used to support it. Generally speaking, successful arguments will rely on well-established facts for their evidence, at least in the beginning. If it’s hard to tell the difference between the claims and the facts, this might be one sign of a badly constructed argument, one that relies on faulty evidence or reasoning to support its claims.
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Representative evidence is absolutely essential. One reason is simple and practical: we rarely have room to include every single example of whatever we’re talking about. Therefore, the examples we do use should be the ones most like all the others—most typical, most representative.
Let’s say I want to prove that Americans are generous, and to prove it I point to Bill Gates. He has given away billions of dollars! Does this prove my claim? Is Gates a typical American? No—he has tons of money to give away, and he has made that a special goal for himself. Therefore, he does not represent Americans in general. He is not the best example. A much better example would be an ordinary working person who makes an average income, because such a person is more likely to represent a typical American.
Researchers go to a lot of trouble to make sure their evidence is representative. For example, in surveys and polls, they work hard to get a random sample of people to talk to. Why? Because picking people at random means you get a typical or representative example. You don’t accidentally limit yourself to members of a certain income group, or ethnicity, or gender, or occupation, or some other category that might distort your results.
Sometimes a single example is all we need, because that one example is completely representative. For instance, if I want to demonstrate the physics of bicycles, I don’t need twenty-five bikes to illustrate my point. One typical bike will do the trick. More often, however, no one example is perfectly representative. We need at least a few to cover the ground. Even then, however, we usually have room for just a small subset of the total, so the ones we choose should be as representative as possible.
This is a major reason why statistics are such an important form of evidence. Many of the subjects we are interested in are way too big to cover with just a few examples. Go back to the question of whether Americans are generous. There are over 300 million Americans, and they are incredibly diverse in age, income, religion, ethnicity, gender, sexuality, political beliefs, region, language, and more. There is no way you could pick enough examples to represent the entire group.
What you can do, however, is give a statistical breakdown. We might say, for example, that 37% of Americans between 31 and 40 years old gave between $100 and $500 to charity in 2009 (Chronicle of Philanthropy, March 21, 2010). No one example, or small group of examples, could accurately represent this complex picture, or tell us how many Americans fit this category.
