November Study Group: Evidence

Every month we also have a study/discussion group about an aspect of critical thinking, logic or argumentation. Our second one in November was about evidence.

“Hitchens’s razor: What can be asserted without evidence can be dismissed without evidence.”


What is evidence?

He who makes an assertion has the burden of proof, or the obligation to produce evidence that leads to a conclusion.  Without evidence, any assertion can, maybe even should, be dismissed.  So what is evidence?  In layman’s terms, evidence is actual tangible, visible objects which lead us to a definitive conclusion about the veracity of something, like a crime.  In reality, evidence can cover much more than this.  Even within the bounds of the law, there are other types of evidence: consider witness testimony or expert evaluation.  Outside of the boundaries of law, philosophy considers such things as experience, observations, states or propositions as evidence.  Evidence verifies or refutes, our beliefs.  When our beliefs are justified, we can then use them as evidence in some cases.

So when we ask “what’s your evidence?” what we’re really asking is “how do you know?” or “what reason do you have to believe?”  When we assert something, we have the “evidential burden”, which is to say it’s up to us to provide evidence to back up our assertions.  Evidence is essentially a means of distinguishing between knowledge and belief, which falls under a branch of philosophy called “epistemology.”  In an argument, the conclusion should necessarily follow the evidence presented.  For the sake of simplicity, we’re not going to get too deep into epistemology, philosophy of science or argumentation theory today.  Instead, we’re going to focus on what evidence is (and isn’t).


Knowledge vs. Belief


First a little on the distinction between knowledge and belief.  While this topic is by no means completely settled, basically belief can be described as a subset of knowledge.  We may believe many things that are not true, but we cannot claim to “know” something that is false.  For example, if a person believes that a bridge is safe enough to support him, and attempts to cross it, but the bridge then collapses under his weight, it could be said that he believed that the bridge was safe but that his belief was mistaken; he didn’t know that it was safe.  In fact, we now know that the bridge was not safe.  By contrast, if the bridge actually supported his weight, then he might say that he believed that the bridge was safe, and now, after proving it to himself (by crossing it), he knows that it is safe.  Knowledge, specifically, is a “justified true” belief, a term coined by Plato, which means that knowledge is belief which has sufficient and justified evidence that makes it true.  So P is true (based on evidence), S believes P is true, therefore S is justified in believing P.  So you could say, the difference between belief and knowledge IS evidence, a stance called “evidentialism.”  When we speak of justification in this context, we are not necessarily speaking of moral or political justification, but rather intellectual, an important distinction.  For instance, while scientists may be intellectually justified in performing experiments on animals, whether or not they are morally justified is a separate question.

According to evidentialism, any two individuals who possessed the same evidence would be equally justified in their beliefs (we will see why this is problematic later).  This is not necessarily the case, though, as will be addressed later.

Without evidence, one can only believe something, not know it.  This, in fact, is often what happens and why I think this topic is so important: people often form beliefs first and then seek out evidence that supports it later, which often results in either cognitive dissonance when contradictory evidence is found, confirmation bias when they selectively search only for evidence which confirms their beliefs or ignorance, intellectual laziness, fallacious beliefs or even dishonesty when they cannot find evidence, or simply don’t bother to.  While a person may feel very certain about a belief, it does not mean they actually know it.  With strong and sufficient evidence, we can call beliefs true and thereby justified, and then it can be called knowledge.  Consider two similar situations: I believe someone has broken into my home.  In the first scenario, nothing was stolen, nothing was moved, my doors and windows are all locked and none are broken.  I have no evidence.  I may vehemently believe my claim, I may feel certain, but do I have cause to?  Not really.  So I can’t accurately say I know someone broke into my home.  On the other hand, if my door was broken open, my room is a mess, things are missing and police find fingerprints of strangers, then with this much evidence, I have good cause to believe my claim.  I could claim that I “know” someone did, particularly if someone was arrested and later convicted of doing so.  In science in particular evidence becomes important in discriminating between scientific theories and speculation or conjecture.  On the basis of evidence, among other things, scientific theories are verified or refuted.

How do we determine what beliefs we are justified in having, or what evidence to adhere to?  There are several theories which I will present very briefly here: Coherentism states that one’s belief should correspond to and not contradict other beliefs they hold.  Foundationalism states that “basic” beliefs, or self-evident beliefs, justify other beliefs.  Externalism states that we should base our beliefs on what is evident externally.  Internalism states that we should base beliefs on our internal knowledge.    While many seem to be opposed to each other, some philosophers have found compatibility between the varying theories.


Now a couple more terms, real quick:


Axioms are statements that all parties involved agree upon.  Loosely, they may simply be statements that two people who are discussing something agree on even if others wouldn’t (like “People want to be happy”).  If they are not agreed upon, then they are no longer axioms and are subject to scrutiny.  More broadly, axioms are statements that people in general agree on, like the existence of reality, the existence of natural laws such as gravity and the constancy of natural laws.  They are the starting points for arguments.  In the broad sense, evidence is not required for axioms; they are part of our shared reality.  On the contrary, if one wishes to reject an axiom, it would require that person to provide evidence for their rejection, as it opposes everything we understand about the universe.

Justification: it is easy to conflate this with an explanation, but they are not the same.  Justification is the reason to hold a belief, or to belief a thing to be knowledge.  It tells us (possibly erroneously) why a belief is true or how one knows a belief is true.  An explanation, on the other hand, simply describes facts to clarify context, cause or consequence.  Science, for instance, is interested in explaining natural phenomenon, like where the universe came from, what forces are at work when you drop an object from a height, or what is going to happen to our universe.  Explanations offer new understanding or facts.  One could use explanations to justify something, but they are not the same thing.  An example: Joe believes Tim’s dog has fleas.  When asked why he thinks so, Joe says Tim’s dog is scratching himself a lot.  That’s justification.  If, based on that belief, Joe and Tim examine Tim’s dog and find out that he does, in fact, have fleas, and Tim asks Joe how dogs get fleas, and Joe suggests that it might be from (the cause might be) the damp weather.  That’s an explanation.  Similarly, a criminal profiler may explain a criminal’s background – for example, by saying that he was recently evicted or grew up in a violent home – but this is not justification.  The criminal profiler may even explain what happened – the criminal broke into someone’s home and stole something, for instance – but this is still not justification.  Justification would be saying WHY a criminal did what he did by, for example, connecting the background to the behavior.

Skepticism: This is another term that gets thrown around and misused a lot.  Typically, when people think of skepticism, they think of stubbornly refusing to believe something, regardless of the evidence.  Skepticism, if anything, is the antithesis of that.  Skepticism is simply an approach to knowledge and belief.  It requires beliefs to be supported by evidence and, if not, that they should be dismissed.  As long as the evidence is available and in accordance with the belief, a skeptic would accept it.  For instance, if I told a skeptic I had a disease but presented no physical symptoms and claimed to have no hospital records, he would reject the claim.  However, if I had skin lesions and documents signed by doctors with diagnosis as well as medicine that I could show him and he could verify is for the particular disease, then he would easily accept it as true.

Self-evidence: if something is self-evident, we do not require outside evidence, we only have to understand the thing to “know” it.  They may be said to be “tautologous”, or true by virtue of form alone, like “What we don’t know is unknown” or “All men are male.”  2+2=4 is a common example.  It is not true “because of” some evidence, it is true simply because the subject and predicate express the same thing.  Therefore, we do not require any external experience or observation or evidence of any kind to verify its truth, only to recognize and understand the concepts.  However, no matter how obvious it may seem not all knowledge is self-evident.  For example, while we may take it for granted nowadays that the earth is round, this is not “self-evident.”  It required gathering evidence before it was believed, but there is no evidence against it, so it is accepted as knowledge.  Earth’s roundness is not self-evident, but it is a fact: it is actually the case or it has actually happened.  In science, this means it is something objective and verifiable.


Ok, back to evidence:


The general rule is that the more evidence I have that corresponds to my belief and the stronger it is, the more likely my beliefs are true.  The less evidence I have, or the more evidence I have that contradicts my beliefs, the less likely.  Consilience – independent sources converging together to verify something – is also important.  It strengthens a claim, and lack of it weakens one.  Consilience is an essential part of science as well as history.  In science, what this means is the more methods that converge on the same conclusions, the stronger the evidence is.  The convergence of geological, biological, psychological and physiological (as well as other) evidence are all strong evidence for evolution, whereas the bible by itself is weaker.  In history, the more varied sources that confirm the same conclusion, the stronger it is.


Propositional Evidence


Claims and assertions are similar; they are statements that something is true or false. We often uses these to justify a belief; whichever belief has the best justification is most likely true.  Evidence leads us to make correct, or at least more likely, conclusions, which lead to correct or more likely true beliefs.  Deductive arguments, or arguments about facts or truth, are necessarily based upon this line of reasoning: If the claims are true, then it follows that the conclusions is also necessarily true.  It is for this reason that evidence is responsible for justified beliefs.  If the evidence demonstrates the beliefs to be true, then we are justified in our knowledge.  As long as our line of thinking relies on a method which has produced more true beliefs than not, we can call it reliable.  We can then often use this same method to come to conclusions about other beliefs as well.


So how much evidence is enough?


So what kind of evidence do we need for what kind of arguments?  The type and scope of evidence necessary depends on the type of claim being made.  The broader and more extraordinary the claim, the more evidence will be necessary.  For example, the claim “I have a pet dog” (a very ordinary and limited claim) should require little evidence.  After all, it’s pretty common to own dogs and it’s only about me.  Whether or not it is true has little to no impact on your or anyone else’s life.  However, if the claim were “I have a pet dragon”, this would require more evidence; it is much more extraordinary and, if true, could impact our reality.  The existence of dragons would raise innumerable questions.

By the same token, saying “At least one swan is white” would require little evidence: it is very limited. It only requires the verification of one swan being white.  “Some swans are white” would require more, since it is broader, while stating that “All swans are white” would require a copious amount of evidence, since it is the widest you can get.  Sometimes we can not assert that a claim is absolutely true, ever, but only that it is more and more likely based on the evidence since it is essentially impossible to affirm the color of all swans that have existed, exist and will ever exist (but this is another problem altogether). Conversely, it is also easier to disprove a claim with such a wide scope.  Even if you had 10,000 examples of white swans, all you would need is an example of ONE black swan – a counterexample – to render such a claim false.  On the other hand, one counterexample would not render the other two claims false.  They are less dependent on large amounts of evidence.  On the contrary, disproving them would be difficult, if not impossible.  In the same vein, if we said, for example, “Humans and monkeys are exactly the same (in all ways)” all we would need is one counterexample to refute such a claim, but saying “humans and monkeys are similar in at least one regard” would be easier to establish but harder to refute.


Our responsibility towards the evidence


Sometimes, however, not only is evidence responsible for justified beliefs, but we must be responsible for our evidence when forming beliefs.  What do I mean by this?  First, it means one must be aware that evidence is evidence and what it is evidence for.  For instance, if I didn’t have anything to drink for a long time and started feeling a headache, but wasn’t aware that headaches were a sign of dehydration, then I would not “know” that I was dehydrated.  I would not know that the evidence (headaches) is, in fact, evidence.  Alternatively, if I assumed it was from stress instead of dehydration, I would be wrong about my conclusion, even though I would be right in saying there was evidence.  I would know that it is evidence, but not what it is evidence for.  In neither case was I being irrational, but my ignorance or faulty reasoning led me to incorrect conclusions.  What this means is we must be in a position to understand the evidence in order to recognize it and come to correct conclusions; we are responsible for our background knowledge.  To this end, two people may have nearly or even exactly the same evidence and come to very different conclusions.  For example, take the social issue of poverty.  While two academics or thinkers may possess exactly the same evidence, due to their background knowledge (as well as biases), one may come to the conclusion that poverty is the fault of psychological states while another may attribute it to social conditions.  One may state that the remedy is more free market while another that it is more government intervention.  This may be the case even if both people are impeccably rational, owing not to bias but lack of knowledge or misuse or misunderstanding of evidence.  Ideally, this would be the case; both parties agree about the evidence.  Who is right?  Based on the available evidence, both.  However, one conclusion must either be correct or more likely than another.  This is dependent on additional evidence and background knowledge.

Evidence and beliefs do not exist in a vacuum.  One cannot come to a conclusion simply because it is based on the conclusion and its evidence; it must be “well-founded” or based on the evidence, rather than one’s preferences.  There are always other beliefs out there, and more evidence out there.  Simply having some evidence for a belief is not sufficient for holding it.  The total sum of evidence out there must be taken into account.  Let’s say I want to believe in alien life.  I may accumulate evidence that suggests alien life is possible, and use that to accurately justify my belief, possibly while ignoring or denying other evidence.  However, let’s say I then state that, based on my belief that alien life is possible, the “rivers” on Mars were probably made by aliens.  This belief is unjustified.  As long as there is contradicting evidence out there, all of it must be weighed before we can be sure of which belief is most likely true.  Evidence, in fact, can be said to be a neutral “arbiter” in determining the truth.  Evidence itself is not predisposed one way or the other. If there is no contradictory evidence, then your position is “indefeasible” and thereby correct.  But there usually is.  For instance, someone may tell me that his name is Bob, but if an associate of his tells me his name is actually Fred (a type of evidence called a “defeater” or “defeating evidence”), then I must take that into consideration as well.  Should better evidence – for example, information that the associate is a pathological liar – arise, that too, must be taken into consideration.  Often, people will stick to unjustified and fallacious beliefs precisely because they refuse to acknowledge evidence outside of their preferred subset.  Of course, being aware of all the possible beliefs and evidence out there is another matter entirely.


Is personal experience evidence?


Beliefs based on personal experience are highly problematic.  For one thing, while we often utilize the testimony of others for trivial things, testimony is prone to several problems and thus weak when it comes to more extraordinary claims.  It may be implausible or lack credibility.  Personal anecdotes can be embellished or even fabricated.  They also provide a very small sample (only one person’s personal experience).  In fact, they often point to idiosyncratic instances, rather than an objective, generalized group of instances and thus are highly unreliable as evidence. Hearsay is worse, as not only does it have the same problems as personal anecdotes, but the conveyer of the information may himself be biased, in error, or have underlying intentions for conveying fallacious information.  Personal experience is also a problem.  We can sometimes be said to be justified in our own beliefs based on experience, but how can we justify it to other people?  Personal perception about, for example, pain or things we see are verified by our own experience of it, but should other people believe based on that?  After all, the pain could be psychosomatic or the vision could be a hallucination.  Our senses are also limited and subject to bias and error.  While we may make some concessions for less extraordinary evidence, like someone telling us they have a headache, or a friend telling us he or she had a bad day at work, when it comes to more extraordinary and impactful claims, such as scientific theories or criminal cases, something which can be independently verified is crucial.  Scientific instruments, such as X-rays, or physical evidence of what you have seen is more objective and less prone to error (not to mention easier to correct) than personal experience.  The same is not true for one’s beliefs, or experiences, or feelings of certainty. If we are to agree that evidence should serve as a neutral arbiter of the truth, then it must be public – accessible to anyone – if it is to remain objective and neutral.  Personal evidence is necessarily subjective and cannot serve this function; it cannot be shared in an objective manner.  Fossil evidence, for example, is stronger evidence of evolution than a personal belief that God exists is for creationism.  By the same token, it is important to use language which is as objective and comprehensible and transmittable – in other words, coherent – as possible when making philosophical claims.  A term like “transcendent” or “soul” is none of these things, but a term like “necessary” is.


A few types of (legal) evidence


If we trace a true belief back to the sources by which we verified its truth, we will find not beliefs, not personal experiences, not perception, but evidence.  In legal terms, the most substantial evidence is called “real” or “material” or “physical” evidence.  It is concrete, objective, neutral and requires little to no inference.  Documentary evidence, while not as strong as physical evidence, is also helpful.  It is essentially information that points to one conclusion or another.  However, it requires inference, which is subject to interpretation problems.  Other types of evidence, such as the above personal experiences, are circumstantial and are quite weak in and of themselves.  Generally, a number of justifications can be produced on account of single circumstantial instances.  However, if compiled with other evidence (producing consilience) – in other words, if they corroborate – then they can become stronger.


Wrap up


That’s it for today.  Keep in mind, you may be labelled as stubborn or close minded or radically skeptical for trying to adhere to the evidence.  You may be accused of “naively” trusting the evidence, and with some cause: evidence can be misleading.  But making mistakes is not the same thing as being irrational.  As long as you are willing to concede that you were in error and make the appropriate changes, you are still being rational.  The more you follow the evidence where it leads, the more correct beliefs you will have, at least compared to someone who ignores or selectively follows evidence.  As W.V. Quine, one of the most prominent logical thinkers, once said, “insofar as we are rational, the intensity of our beliefs will correspond to the firmness of the available evidence.  Insofar as we are rational, we will drop a belief after we have tried in vain to find evidence for it (emphasis added).”  In other words, as long as the evidence – as opposed to our feelings or preferences – lead us to a certain conclusion, we should stick by it.  On the other hand, if it does not lead us to a certain conclusion, no matter how much we may like it or want it to be true, or if there is no evidence for our preferred beliefs, we should drop it.  This is not radical skepticism or close mindedness or stubbornness, this is rationality.  It is the exact opposite of close mindedness and stubbornness.


Axiom: a statement accepted as true as the basis for argument or inference

Belief: a feeling of being sure that something is true

Counter-example: an example that refutes or disproves something

Epistemology: the study or a theory of knowledge

Evidence: something which shows that something else exists or is true

Evidentialism: a theory of justification; states thatthe justification of a belief depends on the evidence for it

Knowledge: something that is known OR the sum of what is known

Reliabilism: a theory of justification; states a belief is justified by whether or not it is related to other beliefs

Skepticism: the method of suspended judgment, doubt, or criticism in regards to a claim


Wikipedia entry:

Simple summary:

More difficult but clear summary:

Highly in-depth look at evidence:


Knowledge and Its Limits by Timothy Williamson

The Book of Evidence by Peter Achinstein

Epistemology: A Contemporary Introduction to the Theory of Knowledge by Robert Audi

Evidentialism, Earl Conee, Richard Feldman


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