There are three main types of evidence you will be using in any argument: Illustrations, Facts, and Inference. Each piece of evidence should substantiate one premise or otherwise solidify your conclusion in the minds of you audience. Several pieces of evidence can be combined to support a premise. 


We'll start at the weakest point: Stories, analogies, comparisons and anecdotes. These types of illustrations can be very helpful in framing your argument. They can be used to firmly plant an idea in your audience's brain and drive home your conclusion. There are many different ways of illustrating an idea. I will only be focusing on two of the most common: anecdotes and analogies. 

Anecdotes, short, personal first- or second-hand stories, can be used, in moderation of course, to explain to your audience how exactly your issue affects a particular person. An anecdote by itself is a very weak tool because it can misrepresent the larger reality or drastically narrow your topic to the point of uselessness. On the other hand, when used properly, in the light of other (more solid) evidence, an anecdote can go a long way toward creating a sympathetic audience.

An analogy is a word picture based on the assumption that if two things are known to be alike in some respects, then they must be alike in other respects. The more numerous the similarities, the stronger the analogy. It is a comparison between two things, usually used to introduce new or complex concepts by comparing them to something that might be familiar to your audience. Analogies are designed to show a relationship between two things, i.e. Puppy is to Dog what Kitten is to Cat. Eventually though an analogy will break down when the number of dissimilarities are pointed out. Similarities are, after all, only likenesses and do not transfer identity: a Puppy will never be a Kitten, even if they both express a number of identical traits. 

While illustrations are powerful tools for illuminating your subject, they are not the best pieces of evidence. If you realize that they are just illustrations, reflections and hints of the reality of your argument and rhetorical devices you will do well. If, on the other hand, you carelessly use them, you will find your argument easy pickings for a critical eye. 

Facts & Figures 

Without facts and statistics, you don't have a leg to stand on in an argument. Facts, whether in the form of statistics, quotations from experts, dictionary or almanac entries, credible news sources, peer-reviewed publications, or linked Web sites, will only stand up to scrutiny if they can pass three tests: verifiability, veracity and relevance. 

Is the fact verifiable? That is, do other sources corroborate your "fact"? If a fact is "common knowledge", then it should be easy to identify at least one source that your audience will agree is reputable. If, more likely, your proposed fact is in dispute, choosing the right source to back up your claim can be problematic. Do you think the G.O.P. Web site would be a better place to find information about President Bush, or would the Democratic National Committee be a better place? Perhaps a more unbiased source would serve you better? Should you, yourself, believe the source in question, and, more importantly, would a reasonably intelligent member of your audience believe it? On that note, however you wish to do it, make sure that all claims of fact are, indeed, referenced somehow. 

Is your "fact" authentic? Is it actually true? Or is it probably true? More often than not, you will have to positively argue the case for the trueness of your "facts". Arguments often turn many of its assumed "facts" into premises; this is a very solid approach, because you can never be sure if your audience will agree with you. 

Finally, is your "fact" completely germane to your argument? It may require some discipline to avoid the irrelevant reasons fallacy. Is it possible to exclude your fact, and still make your case? If so, cut it; there is no need to cloud the issue with extraneous evidence or side arguments. 

Usually, people do not argue over facts, but value, that is whether an action is good, or an object is beautiful. Often, what is in dispute is the meaning of said facts. Here is where you might turn your so-called fact into a premise; defend it, support it, and don't just leave it out there for a quick counter argument. 

Facts alone will not decidedly convince you audience. You might argue that a particular activity is wrong, and demonstrate its negative effect on the country with a whole parade of facts and studies, but in the end you want your audience to be motivated to believe something larger, so you must employ one final piece of evidence. 


Finally, we have our strongest piece of evidence in an argument: inference or what is commonly called "reason". Inference is the most intangible aspect of argumentation; it is also the most important. Inference is the process of bringing your audience through your premises in a logical manner, and then demonstrating that, because your premises are true, your conclusion is true. In a word, inference is logic, without which your facts and illustrations would be essentially meaningless. Take for example, this simple argument: "The teacher failed to correct my paper in a week like he said, therefore I should get an A in the class." The fact that a teacher did not do as he or she promised may be relatively easy to determine and document, but convincing your audience of your conclusion requires the connection of facts with inference that will, hopefully, lead your audience to agree with you. 

Here's a brief outline of a hypothetical demonstration argument:

  1. The syllabus states that the teacher "will grade papers by the day after they are due." (Fact, you can produce the syllabus)
  2. School policy states that teachers will be fired if they do not follow their own syllabus. (Fact, you can produce the district's hiring and firing policy)
  3. School policy states that the student gets a grade of A when the teacher should be fired.  (Fact, you can produce the district's hiring and firing policy on this point too)
  4. Get quotations from other students in the past who did not get their papers back in time.  (Anecdote, you'd have to track down these students)
  5. The teacher's record shows that he had been late in getting papers back in the past.  (Fact, expert testimony, you might have to find a principle or dean as well as obtain the teacher's employment record to show this)
  6. Explain exactly what happened to you. (Anecdote, you are telling your own story.)
  7. The teacher should be fired. (Inference, based on 1, 2)
  8. Therefore, I should get an A in the class.  (Conclusion, what it is you are arguing for.)

What I am calling inference is the process of evaluating, connecting and arranging your facts and illustrations so that your audience will, at the end, agree with your conclusion.  The evidence which you provide is logically connected . . . hopefully.