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You do say!!! Faulty Logic Yields Faulty Results
Category: GENERAL
Tags: logic faulty logic


What is a logical fallacy?
All arguments have the same basic structure: A therefore B. They begin with one or more premises 
(A), which is a fact or assumption upon which the argument is based. They then apply a logical principle (therefore) to arrive at a conclusion (B). An example of a logical principle is that of 
For example, if you begin with the premises that A=B and B=C, you can apply the logical principle of equivalence to conclude that A=C.
A logical fallacy is a false or incorrect logical principle. An argument that is based upon a logical fallacy is therefore not valid. It is important to note that if the logic of an argument is valid then the conclusion must also be valid, which means that if the premises are all true then the conclusion must also be true.
Valid logic applied to one or more false premises, however, leads to an invalid argument. Also, if an argument is not valid the conclusion may, by chance, still be true.
For a more thorough discussion of logical fallacies and how to structure a logical argument, see the New England Skeptical Society's article, How To Argue.
Top 20 Logical Fallacies (in alphabetical order)
- Introduction to Argument
Structure of a Logical Argument
Whether we are consciously aware of it or not, our arguments all follow a certain basic structure. They begin with one or more premises, which are facts that the argument takes for granted as the starting point.
Then a principle of logic is applied in order to 
come to a conclusion. This structure is often illustrated symbolically with the following example: 
Premise1: If A = B, Premise2: and B = C Logical connection: Then (apply principle of equivalence) 
Conclusion: A = C 
In order for an argument to be considered valid the logical form of the argument must work – must 
be valid.
A valid argument is one in which, if the premises are true, then the conclusion must be true also.
However, if one or more premise is false then a valid logical argument may still lead 
to a false conclusion. A sound argument is one in which the logic is valid and the premises are true, in which case the conclusion must be true. 
Also it is important to note that an argument may use wrong information, or faulty logic to reach 
a conclusion that happens to be true.
An invalid or unsound argument does not necessarily prove 
the conclusion false. Demonstrating that an argument is not valid or not sound, however, removes 
it as support for the truth of the conclusion – it means that the conclusion is not necessarily true. 
Breaking down an argument into its components is a very useful exercise, for it enables us to examine both our own arguments and those of others and critically analyze them for validity. This is an excellent way of sharpening one’s thinking, avoiding biases, and making effective arguments. 
Examine your Premises 
As stated above, in order for an argument to be sound all of its premises must be true. Often, different people come to different conclusions because they are starting with different premises. 
So examining all the premises of each argument is a good place to start. 
There are three types of potential problems with premises.
The first, and most obvious, is that a 
premise can be wrong. If one argues, for example, that evolutionary theory is false because there 
are no transitional fossils, that argument is unsound because the premise – no transitional 
fossils – is false. In fact there are copious transitional fossils. 
Another type of premise error occurs when one or more premises is an unwarranted assumption. The 
premise may or may not be true, but it has not been established sufficiently to serve as a premise 
for an argument. Identifying all the assumptions upon which an argument is dependent is often the 
most critical step in analyzing an argument. Frequently, different conclusions are arrived at 
because of differing assumptions. 
Often people will choose the assumptions that best fit the conclusion they prefer. In fact, 
psychological experiments show that most people start with conclusions they desire, then reverse 
engineer arguments to support them – a process called rationalization. 
One way to resolve the problem of using assumptions as premises is to carefully identify and 
disclose those assumptions up front. Such arguments are often called “hypothetical,” or prefaced 
with the statement “Let’s assume for the sake of argument.” Also, if two people examine their 
arguments and realize they are using different assumptions as premises, then at least they can 
“agree to disagree.” They will realize that their disagreement cannot be resolved until more 
information is available to clarify which assumptions are more likely to be correct. 
The third type of premise difficulty is the most insidious: the hidden premise. I have seen this 
listed as a logical fallacy – the unstated major premise, but it is more accurate to consider it 
here. Obviously, if a disagreement is based upon a hidden premise, then the disagreement will be 
irresolvable. So when coming to an impasse in resolving differences, it is a good idea to go back 
and see if there are any implied premises that have not been addressed. 
Let’s go back to the transitional fossil example again. Why is it that scientists believe we have 
many transitional fossils and evolution deniers (creationists or intelligent design proponents) 
believe that we do not. This would seem to be a straightforward factual claim easily resolvable by 
checking the evidence. Sometimes evolution deniers are simply ignorant of the evidence or are 
being intellectually dishonest. However, the more sophisticated are fully aware of the fossil 
evidence and use a hidden premise to deny the existence of transitional fossils. 
When a paleontologist speaks of “transitional” fossils, they are referring to species that occupy 
a space morphologically between two known species. This may be a common ancestor, in which case 
the transitional fossil will be more ancient than both descendant species; or it can be temporally 
between two species, the descendant of one and the ancestor of the other. But in reality we often 
do not know if the transitional species is an actual ancestor or just closely related to the true 
ancestor. Because evolution is a bushy process, and not linear, most of the specimens we find will 
lie on an evolutionary side branch (an uncle rather than a parent). But if they fill a 
morphological gap in known species, they provide evidence of an evolutionary connection, and 
therefore qualify as transitional. For example, archaeopteryx may not be on the direct path to 
modern birds, but clearly they occupy a space that is transitional between therapod dinosaurs and 
modern birds and one of their close relatives is a direct ancestor to modern birds. 
When evolution deniers say there are no transitional fossils their unstated major premise is that 
they are employing a different definition of transitional than is generally accepted in the 
scientific community. They typically define transitional as some impossible monster with 
half-formed and useless structures. Or, they may define transitional as only those fossils for 
which there is independent proof of being a true ancestor, rather than simply closely related to a 
direct ancestor – an impossible standard. 
Another hidden premise in their argument is the notion of how many transitional fossils there 
should be in the fossil record. They, of course, can always assume an arbitrarily high number to 
claim that there isn’t enough. 
- Introduction to Logical Fallacies
Even when all of the premises of an argument are reliably true, the argument may still be invalid 
if the logic employed is not legitimate – a so-called logical fallacy. The human brain is a 
marvelous machine with capabilities that, in some ways, still outperform the most powerful of 
super computers. Our brains, however, do not appear to have evolved specifically for precise 
logic. There are many common logical pitfalls that our minds tend to fall into, unless we are 
consciously aware of these pitfalls and make efforts to avoid them. 
Humans also tend to use logical short-cuts, called heuristics. These are thought processes that 
are not strictly valid in their logic, but are true most of the time and therefore are a useful 
rule-of-thumb as to what is likely to be true. But they get us into trouble when they substitute 
for valid logic. 
Also because, as stated above, there is a tendency to start with desired conclusions and then 
construct arguments to support them, many people will happily draw upon logical fallacies to make 
their arguments. In fact, if a conclusion is not true one must either employ a false premise or a 
logical fallacy in order to construct an argument that leads to that conclusion. Remember, a sound 
argument (one with true premises and valid logic) cannot lead to a false conclusion. So in order 
to avoid using logical fallacies to construct invalid arguments, we need to understand how to 
identify fallacious logic. 
Below I will list the most common logical fallacies, with examples of each. 
On a side note, I have found many lists of logical fallacies, and they tend to differ along the 
“lumper vs splitter” spectrum. Many fallacies are really just specific subtypes of a more general 
fallacy. I have taken a combined approach, listing the main types of fallacies and giving examples 
of subtypes where appropriate. 
Although this list started as our "top 20 logical fallacies," we include more than 20 fallacies 
for your information, and will likely continue to add to the list. 
Ad hominem
An ad hominem argument is any that attempts to counter another’s claims or conclusions by 
attacking the person, rather than addressing the argument itself. True believers will often commit 
this fallacy by countering the arguments of skeptics by stating that skeptics are closed minded. 
Skeptics, on the other hand, may fall into the trap of dismissing the claims of UFO believers, for 
example, by stating that people who believe in UFO’s are crazy or stupid. 
A common form of this fallacy is also frequently present in the arguments of conspiracy theorists 
(who also rely heavily on ad-hoc reasoning). For example, they may argue that the government must 
be lying because they are corrupt. 
It should be noted that simply calling someone a name or otherwise making an ad hominem attack is 
not in itself a logical fallacy. It is only a fallacy to claim that an argument is wrong because 
of a negative attribute of someone making the argument. (i.e. “John is a jerk.” is not a fallacy. 
“John is wrong because he is a jerk.” is a logical fallacy.) 
The term “poisoning the well” also refers to a form of ad hominem fallacy. This is an attempt to 
discredit the argument of another by implying that they possess an unsavory trait, or that they 
are affiliated with other beliefs or people that are wrong or unpopular. A common form of this 
also has its own name – Godwin’s Law or the reductio ad Hitlerum. This refers to an attempt at 
poisoning the well by drawing an analogy between another’s position and Hitler or the Nazis. 
Ad ignorantiam
The argument from ignorance basically states that a specific belief is true because we don’t know 
that it isn’t true. Defenders of extrasensory perception, for example, will often overemphasize 
how much we do not know about the human brain. It is therefore possible, they argue, that the 
brain may be capable of transmitting signals at a distance. 
UFO proponents are probably the most frequent violators of this fallacy. Almost all UFO eyewitness 
evidence is ultimately an argument from ignorance – lights or objects sighted in the sky are 
unknown, and therefore they are alien spacecraft. 
Intelligent design is almost entirely based upon this fallacy. The core argument for intelligent 
design is that there are biological structures that have not been fully explained by evolution, 
therefore a powerful intelligent designer must have created them. 
In order to make a positive claim, however, positive evidence for the specific claim must be 
presented. The absence of another explanation only means that we do not know – it doesn’t mean we 
get to make up a specific explanation. 
Argument from authority
The basic structure of such arguments is as follows: Professor X believes A, Professor X speaks 
from authority, therefore A is true. Often this argument is implied by emphasizing the many years 
of experience, or the formal degrees held by the individual making a specific claim. The converse 
of this argument is sometimes used, that someone does not possess authority, and therefore their 
claims must be false. (This may also be considered an ad-hominen logical fallacy – see below.) 
In practice this can be a complex logical fallacy to deal with. It is legitimate to consider the 
training and experience of an individual when examining their assessment of a particular claim. 
Also, a consensus of scientific opinion does carry some legitimate authority. But it is still 
possible for highly educated individuals, and a broad consensus to be wrong – speaking from 
authority does not make a claim true. 
This logical fallacy crops up in more subtle ways also. For example, UFO proponents have argued 
that UFO sightings by airline pilots should be given special weight because pilots are trained 
observers, are reliable characters, and are trained not to panic in emergencies. In essence, they 
are arguing that we should trust the pilot’s authority as an eye witness. 
There are many subtypes of the argument from authority, essentially referring to the implied 
source of authority. A common example is the argument ad populum – a belief must be true because 
it is popular, essentially assuming the authority of the masses. Another example is the argument 
from antiquity – a belief has been around for a long time and therefore must be true. 
Argument from final Consequences
Such arguments (also called teleological) are based on a reversal of cause and effect, because 
they argue that something is caused by the ultimate effect that it has, or purpose that is serves. 
Christian creationists have argued, for example, that evolution must be wrong because if it were 
true it would lead to immorality. 
One type of teleological argument is the argument from design. For example, the universe has all 
the properties necessary to support live, therefore it was designed specifically to support life 
(and therefore had a designer. 
Argument from Personal Incredulity
I cannot explain or understand this, therefore it cannot be true. Creationists are fond of arguing 
that they cannot imagine the complexity of life resulting from blind evolution, but that does not 
mean life did not evolve. 
Begging the Question
The term “begging the question” is often misused to mean “raises the question,” (and common use 
will likely change, or at least add this new, definition). However, the intended meaning is to 
assume a conclusion in one’s question. This is similar to circular reasoning, and an argument is 
trying to slip in a conclusion in a premise or question – but it is not the same as circular 
reasoning because the question being begged can be a separate point. Whereas with circular 
reasoning the premise and conclusion are the same. 
The classic example of begging the question is to ask someone if they have stopped beating their 
wife yet. Of course, the question assumes that they every beat their wife. 
In my appearance on the Dr. Oz show I was asked – what are alternative medicine skeptics (termed 
“holdouts”) afraid of? This is a double feature of begging the question. By using the term 
“holdout” the question assumes that acceptance is already become the majority position and is 
inevitable. But also, Oz begged the question that skeptics are “afraid.” This also created a straw 
man (see below) of our position, which is rather based on a dedication to reasonable standards of 
science and evidence. 
Confusing association with causation
This is similar to the post-hoc fallacy in that it assumes cause and effect for two variables 
simply because they occur together. This fallacy is often used to give a statistical correlation a 
causal interpretation. For example, during the 1990’s both religious attendance and illegal drug 
use have been on the rise. It would be a fallacy to conclude that therefore, religious attendance 
causes illegal drug use. It is also possible that drug use leads to an increase in religious 
attendance, or that both drug use and religious attendance are increased by a third variable, such 
as an increase in societal unrest. It is also possible that both variables are independent of one 
another, and it is mere coincidence that they are both increasing at the same time. 
This fallacy, however, has a tendency to be abused, or applied inappropriately, to deny all 
statistical evidence. In fact this constitutes a logical fallacy in itself, the denial of 
causation. This abuse takes two basic forms. The first is to deny the significance of correlations 
that are demonstrated with prospective controlled data, such as would be acquired during a 
clinical experiment. The problem with assuming cause and effect from mere correlation is not that 
a causal relationship is impossible, it’s just that there are other variables that must be 
considered and not ruled out a-priori. A controlled trial, however, by its design attempts to 
control for as many variables as possible in order to maximize the probability that a positive 
correlation is in fact due to a causation. 
Further, even with purely epidemiological, or statistical, evidence it is still possible to build 
a strong scientific case for a specific cause. The way to do this is to look at multiple 
independent correlations to see if they all point to the same causal relationship. For example, it 
was observed that cigarette smoking correlates with getting lung cancer. The tobacco industry, 
invoking the “correlation is not causation” logical fallacy, argued that this did not prove 
causation. They offered as an alternate explanation “factor x”, a third variable that causes both 
smoking and lung cancer. But we can make predictions based upon the smoking causes cancer 
hypothesis. If this is the correct causal relationship, then duration of smoking should correlate 
with cancer risk, quitting smoking should decrease cancer risk, smoking unfiltered cigarettes 
should have a higher cancer risk than filtered cigarettes, etc. If all of these correlations turn 
out to be true, which they are, then we can triangulate to the smoking causes cancer hypothesis as 
the most likely possible causal relationship and it is not a logical fallacy to conclude from this 
evidence that smoking probably causes lung cancer.
Confusing currently unexplained with unexplainable
Because we do not currently have an adequate explanation for a phenomenon does not mean that it is 
forever unexplainable, or that it therefore defies the laws of nature or requires a paranormal 
explanation. An example of this is the "God of the Gapsa" strategy of creationists that whatever 
we cannot currently explain is unexplainable and was therefore an act of god. 
False Analogy
Analogies are very useful as they allow us to draw lessons from the familiar and apply them to the 
unfamiliar. Life is like a box of chocolate – you never know what you’re going to get. 
A false analogy is an argument based upon an assumed similarity between two things, people, or 
situations when in fact the two things being compared are not similar in the manner invoked. 
Saying that the probability of a complex organism evolving by chance is the same as a tornado 
ripping through a junkyard and created a 747 by chance is a false analogy. Evolution, in fact, 
does not work by chance but is the non-random accumulation of favorable changes. 
Creationists also make the analogy between life and your home, invoking the notion of 
thermodynamics or entropy. Over time your home will become messy, and things will start to break 
down. The house does not spontaneously become more clean or in better repair. 
The false analogy here is that a home is an inanimate collection of objects. Whereas life uses 
energy to grow and reproduce – the addition of energy to the system of life allows for the local 
reduction in entropy – for evolution to happen. 
Another way in which false analogies are invoked is to make an analogy between two things that are 
in fact analogous in many ways – just not the specific way being invoked in the argument. Just 
because two things are analogous in some ways does not mean they are analogous in every way. 
False Continuum
The idea that because there is no definitive demarcation line between two extremes, that the 
distinction between the extremes is not real or meaningful: There is a fuzzy line between cults 
and religion, therefore they are really the same thing. 
False Dichotomy
Arbitrarily reducing a set of many possibilities to only two. For example, evolution is not 
possible, therefore we must have been created (assumes these are the only two possibilities). This 
fallacy can also be used to oversimplify a continuum of variation to two black and white choices. 
For example, science and pseudoscience are not two discrete entities, but rather the methods and 
claims of all those who attempt to explain reality fall along a continuum from one extreme to the 
Genetic Fallacy
The term “genetic” here does not refer to DNA and genes, but to history (and therefore a 
connection through the concept of inheritance). This fallacy assumes that something’s current 
utility is dictated by and constrained by its historical utility. This is easiest to demonstrate 
with words – a words current use may be entirely unrelated to its etymological origins. For 
example, if I use the term “sunset” or “sunrise” I am not implying belief in a geocentric 
cosmology in which the sun revolves about the Earth and literally “rises” and “sets.” 
Applying criteria or rules to one belief, claim, argument, or position but not to others. For 
example, some consumer advocates argue that we need stronger regulation of prescription drugs to 
ensure their safety and effectiveness, but at the same time argue that medicinal herbs should be 
sold with no regulation for either safety or effectiveness. 
No True Scotsman
This fallacy is a form of circular reasoning, in that it attempts to include a conclusion about 
something in the very definition of the word itself. It is therefore also a semantic argument. 
The term comes from the example: If Ian claims that all Scotsman are brave, and you provide a 
counter example of a Scotsman who is clearly a coward, Ian might respond, "Well, then, he's no 
true Scotsman." In essence Ian claims that all Scotsman are brave by including bravery in the 
definition of what it is to be a Scotsman. This argument does not establish and facts or new 
information, and is limited to Ian's definition of the word, "Scotsman." 
In Latin this term translates to "doesn't follow". This refers to an argument in which the 
conclusion does not necessarily follow from the premises. In other words, a logical connection is 
implied where none exists. 
Post-hoc ergo propter hoc
This fallacy follows the basic format of: A preceded B, therefore A caused B, and therefore 
assumes cause and effect for two events just because they are temporally related (the latin 
translates to "after this, therefore because of this"). 
Reductio ad absurdum
In formal logic, the reductio ad absurdum is a legitimate argument. It follows the form that if 
the premises are assumed to be true it necessarily leads to an absurd (false) conclusion and 
therefore one or more premises must be false. The term is now often used to refer to the abuse of 
this style of argument, by stretching the logic in order to force an absurd conclusion. For 
example a UFO enthusiast once argued that if I am skeptical about the existence of alien visitors, 
I must also be skeptical of the existence of the Great Wall of China, since I have not personally 
seen either. This is a false reductio ad absurdum because he is ignoring evidence other than 
personal eyewitness evidence, and also logical inference. In short, being skeptical of UFO’s does 
not require rejecting the existence of the Great Wall. 
Slippery Slope
This logical fallacy is the argument that a position is not consistent or tenable because 
accepting the position means that the extreme of the position must also be accepted. But moderate 
positions do not necessarily lead down the slippery slope to the extreme. 
Special pleading, or ad-hoc reasoning
This is a subtle fallacy which is often difficult to recognize. In essence, it is the arbitrary 
introduction of new elements into an argument in order to fix them so that they appear valid. A 
good example of this is the ad-hoc dismissal of negative test results. For example, one might 
point out that ESP has never been demonstrated under adequate test conditions, therefore ESP is 
not a genuine phenomenon. Defenders of ESP have attempted to counter this argument by introducing 
the arbitrary premise that ESP does not work in the presence of skeptics. This fallacy is often 
taken to ridiculous extremes, and more and more bizarre ad hoc elements are added to explain 
experimental failures or logical inconsistencies. 
Straw Man
A straw man argument attempts to counter a position by attacking a different position – usually 
one that is easier to counter. The arguer invents a caricature of his opponent’s position – a 
“straw man” – that is easily refuted, but not the position that his opponent actually holds. 
For example, defenders of alternative medicine often argue that skeptics refuse to accept their 
claims because they conflict with their world-view. If “Western” science cannot explain how a 
treatment works, then it is dismissed out-of-hand. If you read skeptical treatment of so-called 
“alternative” modalities, however, you will find the skeptical position much more nuanced than 
Claims are not a-prior dismissed because they are not currently explained by science. Rather, in 
some cases (like homeopathy) there is a vast body of scientific knowledge that says that 
homeopathy is not possible. Having an unknown mechanism is not the same thing as demonstrably 
impossible (at least as best as modern science can tell). Further, skeptical treatments of 
homeopathy often thoroughly review the clinical evidence. Even when the question of mechanism is 
put aside, the evidence shows that homeopathic remedies are indistinguishable from placebo – which 
means they do not work. 
Tautology in formal logic refers to a statement that must be true in every interpretation by its 
very construction. In rhetorical logic, it is an argument that utilizes circular reasoning, which 
means that the conclusion is also its own premise. Typically the premise is simply restated in the 
conclusion, without adding additional information or clarification. The structure of such 
arguments is A=B therefore A=B, although the premise and conclusion might be formulated 
differently so it is not immediately apparent as such. For example, saying that therapeutic touch 
works because it manipulates the life force is a tautology because the definition of therapeutic 
touch is the alleged manipulation (without touching) of the life force. 
The Fallacy Fallacy
As I mentioned near the beginning of this article, just because someone invokes an unsound 
argument for a conclusion, that does not necessarily mean the conclusion is false. A conclusion 
may happen to be true even if an argument used to support is is not sound. I may argue, for 
example, Obama is a Democrat because the sky is blue – an obvious non-sequitur. But the 
conclusion, Obama is a Democrat, is still true. 
Related to this, and common in the comments sections of blogs, is the position that because some 
random person on the internet is unable to defend a position well, that the position is therefore 
false. All that has really been demonstrated is that the one person in question cannot adequately 
defend their position. 
This is especially relevant when the question is highly scientific, technical, or requires 
specialized knowledge. A non-expert likely does not have the knowledge at their fingertips to 
counter an elaborate, but unscientific, argument against an accepted science. “If you (a lay 
person) cannot explain to me,” the argument frequently goes, “exactly how this science works, then 
it is false.” 
Rather, such questions are better handled by actual experts. And, in fact, intellectual honesty 
requires that at least an attempt should be made to find the best evidence and arguments for a 
position, articulated by those with recognized expertise, and then account for those arguments 
before a claim is dismissed. 
The Moving Goalpost
A method of denial arbitrarily moving the criteria for “proof” or acceptance out of range of 
whatever evidence currently exists. If new evidence comes to light meeting the prior criteria, the 
goalpost is pushed back further – keeping it out of range of the new evidence. Sometimes 
impossible criteria are set up at the start – moving the goalpost impossibly out of range -for the 
purpose of denying an undesirable conclusion. 
Tu quoque
Literally, you too. This is an attempt to justify wrong action because someone else also does it. 
"My evidence may be invalid, but so is yours."
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