Evidence. The heart of any debate. The deciding factor in court. The raw material for argumentation. What is it? What is it not? This essay is designed to help the reader learn to categorize evidence, rank its strength, and critically examine opposing evidence.

Evidence, strictly speaking, consists of facts, opinions, and actual objects. This seems clear enough at first glance, but it's important to understand a couple of things about facts and opinions. We all know the saying, "Opinions are like assholes. Everybody's got one." This is normally used to say that a particular opinion is not pertinent to a conversation, but it gives an impression that is incorrect. All opinions are not of equal value. In matters of speculation, the opinion of a scholarly expert is far more valuable than that of an untrained novice. This is why you see "expert witnesses" in court as opposed to "Run of the mill every day witnesses." (Of course, the debate over whether or not all expert witnesses are actually experts is a totally different matter. This essay assumes that experts are what they claim to be.) Opinions do, in fact, have a place in debate. It's not always about fact, and sometimes, preponderance of opinion is a crucial issue. We will explore this more later.

Direct and Presumptive Evidence.
Direct evidence is evidence that is sufficient to demonstrate the fact in question without the need to question any of the supporting facts. In a debate about the legality of abortion, a simple reference from the Supreme Court Records would establish without question that the case was heard and decided between 1971 and 1973. No further fact checking is necessary, and it is not necessary to establish the credibility of the Supreme Court records. Direct evidence seldom supports the proposition, for if there were such direct evidence, the proposition would hardly be debatable. So, direct evidence appears almost exclusively in support of contentions.

Presumptive evidence, also known as indirect or circumstantial, is evidence that tends to demonstrate a fact in question by inference. Clearly, presumptive evidence is not as strong as direct evidence, and when the two are at odds, direct evidence always prevails. Suppose that someone claimed to see a UFO over the White House at 6PM on Friday. Suppose further that this person was a congressman, and a well trusted confidant of the President. Suppose that in addition, several of his most trusted aides were with him, and also claim to have seen the UFO. This could be considered strong circumstantial evidence, and a debate might well end with the conclusion that there was a UFO over the White House. However, if there was file tape from Friday, with a direct, panoramic view of the sky over the White House, and no UFO appeared, the direct evidence would prevail, and the conclusion would have to be that there was no UFO over the White House.

One more note about presumptive evidence: The claim that you can't convict someone on circumstantial evidence alone is patently false. Cases with direct evidence of guilt seldom go to trial. Think about it. If there is clear, direct evidence, such as a videotape of a crime being committed, the case seldom even makes it to trial. The defendant will usually try to negotiate a plea bargain before it ever gets to a judge. The reality is that most cases involve direct evidence in support of contentions. Enough proven contentions provide enough presumptive evidence to convict someone.

Primary and Secondary Evidence.
Primary evidence is original evidence. The original text of a book, an actual fossil, or a bloody knife would all be considered primary evidence. This is, of course, the most certain kind of evidence, and is always favored over secondary evidence, in much the same way as direct evidence always takes precedence over presumptive evidence.

Secondary evidence, then, is anything that is not primary. Third generation copies of an ancient text, a photo of a fossil, or an artist's rendering of a court room are all secondary evidence. Clearly, secondary evidence carries much less weight, and demands far more corroboration before it can be considered strong enough to carry a proposition.

Real and Personal Evidence
Real evidence is essentially object(s) that demonstrate a claim. Fingerprints, scars, bloody shoes, maps, and videotape are all real evidence. Since it is extremely difficult to argue against the existence of something that is sitting on a table in front of you, real evidence is some of the strongest evidence in any debate. It must be noted that real evidence can be used to support any part of a contention or proposition. In other words, the contention that there is such a thing as a beetle that looks just like an ant would be directly proven by the presence of just such a beetle in a display case for everyone to see. The proposition under debate, however, might be the evolutionary path taken by animals which mimic other animals. In this case, the beetle would be just one of many pieces to a much larger puzzle.

Personal evidence, as the name implies, is any evidence furnished by a person. For the purpose of internet debate, this is always essentially written testimony. Of course, in the outside world, there is oral evidence, too. As you might expect, personal evidence is considered very weak unless there is corroborating real evidence.

Lay and Expert Evidence.
As promised, I will devote a small amount of space to lay and expert evidence. Anyone without special training is considered a layperson, and their opinion is considered a lay opinion. People with specific, relevant training are considered experts, as are their opinions. Clearly, expert opinion carries substantially more weight than lay opinion. Two words of caution, though: Opinions are still opinions, and facts are still facts. Facts always win, no matter how expert the opinion. Also, expert opinion only carries more weight in the area of expertise! A biologist is not qualified to give an expert opinion on physics, and a philosopher has no business portraying his opinion of cosmology as expert.

Naturally, there are different levels of "expert." Stephen Hawking is one of the foremost experts on cosmology in the world. Someone with an undergraduate degree in theoretical physics is also an expert, although it is extremely important to recognize the limits of a person's expertise. The college graduate's ability to give evidence in theoretical physics only extends as far as his training. If he's never studied the black hole information paradox, he's not qualified to give evidence about it. At a very simple level, someone could be considered an expert witness for a trivial matter. Since I have read "Climbing Mount Improbable" and have a copy on my desk, I could be considered an expert witness if the question was whether or not the book addresses the evolution of the eye.

Although I will be covering fallacious arguments in another essay, this is a good time to give a cursory nod towards the fallacy of "argument from authority." Argument from authority is not always fallacious. When a matter can be simply settled by consulting an expert, argument from authority is usually the easiest way to settle it. If you were trying to support the proposition that many composers in France were fond of Le Chat Noir in the 19th century, you could simply ask a music historian, and the question would be settled adequately. Where an appeal to authority becomes fallacious is when it is used to cover over a lack of supporting evidence, or when it is essentially a presumptive opinion used to make a position appear more authoritative without contributing any real information.

Partial Proof, Corroborative Proof, Indispensable Proof, and Conclusive Proof
Partial proof is evidence that supports a single element within a larger contention. City tax records indicating that bars and restaurants accounted for 20% of a city's business tax income would partially prove the contention that bars and restaurants were a beneficial and important part of that city's tourist industry.

Corroborative proof is strengthening evidence. In debating the question of whether or not immigrant workers are harming the local work force, a single case of an American worker being put out of work would be corroborating evidence, although it would not be very strong evidence. Five hundred individual cases would be much stronger corroborating evidence. In this way, corroboration is cumulative, and gains strength in numbers.

Indispensable proof is evidence without which a particular claim cannot be proved. Obviously, an entire proposition fails if indispensable proof is missing from the positive claimant's supporting evidence.

Conclusive proof is evidence that is incontrovertible. What constitutes incontrovertibility varies according to the contention. This one topic could be another essay entirely, but I will leave it as it stands for the time being. There are numerous essays on this site concerning the nature of knowledge and what constitutes certainty. A little digging will go a long way if you're interested.

For my last topic in this essay, I want to mention briefly the epistemology of the words "belief" and "knowledge." These are often used somewhat haphazardly, but for productive debate, it's important to note the distinction. Philosophers are not in complete agreement about what, exactly, knowledge is. I don't claim to be presenting an authoritative and final definition. However, the following definition can be considered complete and authoritative for the purpose of critical thinking. Knowledge, simply put, is justified true belief. What this means is that only things that are objectively true are knowledge. Furthermore, even beliefs that are true, but lack justification, are not truly knowledge! I believe that Georgia will defeat South Carolina this weekend in college football. Even if it turns out that my belief is correct, it would not be correct to say that I know it now. There is not enough justification to move it from belief to knowledge. Time doesn't have to have anything to do with it, either. I don't know for certain that there is a crossword in today's Atlanta Journal Constitution. There have been days when it has been omitted, and even though I feel certain that it will be there when I open the Living section, I won't technically know it to be true until I see it firsthand.

Belief, then, is partially justified, and not necessarily true. Anything that does not have enough justification to be essentially 100% certain is a belief. This can get tricky, as anyone who has studied philosophy can attest. Technically speaking, outside of the axiom of identity and its corollaries, it can be said that nothing can be known with 100% certainty. The reality, of course, is that we take many things to be 100% certain based on the overwhelming and irrefutable evidence that they are so. I am 100% certain that my computer exists, as you are of yours. You couldn't be reading this if I didn't exist, so you are also 100% certain of my existence. This essay is not addressing the philosophical questions of inductive and deductive certainty. We will henceforth assume that you and I both operate on the belief that the world is as it appears, and that we are not dreaming, deceived, or insane.

The reason I have digressed into belief and knowledge is that these concepts are crucial to good critical thinking skills. In order to be able to arrive at true conclusions, and to examine your own ideas objectively, it is paramount that you are able to separate knowledge and belief, and treat them accordingly. Any belief can be wrong, and should be treated as suspect. Outside of our direct observations, we actually know very little, and it is good to remember this when debating others, and more importantly, when examining our own beliefs!

Next, how to examine the validity of evidence.