Logical Fallacies


In this article I will discuss at length, the very nature of argumentation and proper methods for achieving sound conclusions. This document reads somewhat like a technical manual, but to be confident in our conclusions it is essential that we give much care in developing an approach to information that is sound. It should be noted briefly that logic is not an ideology, nor is it biased toward anything besides proper formulation of conclusions. In short, logic is the 'science of reasoning, proof, thinking, or inference' (Oxford English Dictionary).

I will at some time in the future be adding information on renowned logicians. Check back for that.

Understanding Arguments

This first thing we must understand, is what an argument is. An argument is a statement containing one or more premises and a conclusion. Second we must understand what a premise is. A premise is a statement (information, usually a sentence; that is either true or false) that's intent is to support the claim being made, which is the conclusion (which is also information that is true or false).

Types of Arguments

There are two primary forms of argumentation. Inductive and deductive.
  • A deductive argument is an argument in which the premises are intended to completely support and provide sufficient evidence for the conclusion.
  • An inductive argument is an argument in which the premises are intended to provide some degree, or probable support for the conclusion.
There are two measures of an argument's soundness

1. Inferential Relationship (how much support does the premise provide for the conclusion

If the premises were true, would the conclusion necessarily (deductive), or probably (inductive), be true?

2. The truth of the premises.

If the premises actually provide the required degree of support for the conclusion, then the argument is a good one. We use logic to determine this.

Sorting Arguments

 The basis for a valid deductive argument is one in which if all of it's premises are true, the conclusion must be true (cf. Inferential Relationship). The validity of a deductive argument does not depend on the premises or conclusion being true, which is to say that even if all premises and the conclusion are false, the deductive argument may be valid in form. Another way of stating this validity, is to say that if the conclusion is false, one of the premises must also be false for the deductive argument to be considered invalid. If the deductive argument is valid, and has all true premises, then it becomes logically sound.

Below is a symbolic representation of good inferential Relationship in a valid deductive argument. The symbols used here mean "if and only if" [ W ].

My formula for a valid and sound deductive argument is as follows

T=True, F=False, V=Valid, S=Sound, Q = x and y ↔ N; V(Q), x or y→F then ¬S(Q); x and y→T ↔ S(Q)

2. The basis for a strong inductive argument is one in which if it's premises are true, then it must be likely that the conclusion is true (cf. Inferential Relationship). In this scenario, if the conclusion is false, one or more of the premises is probably false. Finally, if the inductive argument is strong and the premises are true, the inductive argument is 

Cohesion in thought / Rational thought

In general terms, a logically fallacy is an error in reasoning. In short, there is a correct way and an incorrect way to form conclusions, and indeed to argue them. Knowledge of fallacies provides an object rule for gauging the rationality of thought and argumentation. An inability to reconcile with logic, especially when a fallacy is called by name is a demonstrable act of irrationality.

Logical Fallacies, the measure of rationality.

When an argument is made that commits a logical fallacy, this means that the argument lacks sufficient evidence to be true based on the argument being made. This is why one of the most essential tools in anyone's intellectual arsenal is the ability to perform on the spot logical evaluations of arguments. Honing theses skills equip the thinker with the ability to distinguish correctly formulated arguments from irrational statements. The goal being advancement of knowledge through learning and indeed debate. Logic is the object rule of both Science and Theology, and without it neither can stand.

Again, the truth of logical fallacies comes from the scientific method. An argument is a logical fallacy when it is demonstrable that the argument made does not support that the conclusion is true.

Deductive or Inductive fallacies

a. A deductive fallacy is a deductive argument that is invalid, or logically fallacious. The arguments premises could be all true but the conclusion is erroneous.

b. An inductive fallacy is less formal than a deductive fallacy. They are simply "arguments" which appear to be inductive arguments, but the premises do not provided sufficient evidence for the conclusion. In such cases, even if the premises were true, the conclusion is not more likely to be true.

Below is an explanation of some of the more common logical fallacies and how they occur.

Formal Fallacies

Argumentum ad hominem

This fallacy occurs when the respondandant avoids the substance of the original argument or avoids producing evidence to the contrary and instead attacks the argument itself, by attacking or appealing to an alleged characteristic or belief of the source making the argument.


"You claim that this man is innocent, but you cannot be trusted since you are a criminal as well."

Inverse Ad Hominem

An inverse ad hominem argument praises a source in order to add support for that source's argument or claim.

  • "These are decent people. They live good lives and have good morals. Therefore their arguments should be accepted as true."
  • "That man was smartly-dressed and charming, so I'll accept his argument that I should vote for him."
Another type of Inverse ad hominem is the ad verecundiam, or appeal to authority (see below).

Argumentum ad hominem personam, ad hominem abusive

Ad hominem Personam (also called argumentum ad personam or ad hominem abusive) usually and most notoriously involves insulting or belittling one's opponent, but can also involve pointing out factual but ostensibly "damning" character flaws or actions which are irrelevant to the opponent's argument. This tactic is logically fallacious because insults and even true negative facts about the opponent's personal character have nothing to do with the logical merits of the opponent's arguments or assertions.

  • "You can't believe Jack when he says God exists. He doesn't even have a job."
  • "Candidate Jane's proposal about zoning is ridiculous. She was caught cheating on her taxes in 2003."

Argumentum Ad Hominem Tu Quoque

This fallacy occurs when the statement put forth is argued against on the basis that the person's previous actions or statements contradict their current argument.

John: Smoking is bad, it can kill you.
Jane: How can you say that when you're a smoker?

Jane committed the logical fallacy.

Other Examples
  • "You say that stealing is wrong, but you do it as well."
  • "He says we shouldn't enslave people, yet he himself owns slaves"

Argumentum Ad Hominem Circumstantialis

Ad Hominem Circumstantialis (Circumstantial) involves pointing out that someone is in circumstances such that he is disposed to take a particular position. Essentially, ad hominem circumstantial constitutes an attack on the bias of a source. The reason that this is fallacious in syllogistic logic is that pointing out that one's opponent is disposed to make a certain argument does not make the argument, from a logical point of view, any less credible; this overlaps with the genetic fallacy (an argument that a claim is incorrect due to its source).

  • "Of course your support the Pope's view, you're Catholic!"

Ad Verecundiam, Ipse Dixit, Appeal to Authority

This fallacy bases the truth value of an assertion on the authority, knowledge, expertise, or position of the source asserting it. It is also known as argument from authority, argumentum ad verecundiam (Latin: argument to respect) or ipse dixit (Latin: "he himself said it").


  1. A makes claim B.
  2. There is something positive about A.
  3. Therefore it is concluded that claim B is true.
"My teacher said so, therefore it must be so."

Argumentum Ad Novitatem (Appeal to Novelty)

The appeal to novelty (also called argumentum ad novitatem) is a fallacy in which someone prematurely claims that an idea or proposal is correct or superior, exclusively because it is new and modern. In a controversy between status quo and new inventions, an appeal to novelty argument isn't in itself a valid argument. The fallacy may take two forms: overestimating the new and modern, prematurely and without investigation assuming it to be best-case, or underestimating status quo, prematurely and without investigation assuming it to be worst-case.

  • "If you want to lose weight, your best bet is to follow the latest diet."
  • "The department will become more profitable because it has been reorganized."
  • "Upgrading all your software to the most recent versions will make your system more reliable."
  • "Things are bad with party A in charge, thus party B will bring improvement if they're elected."
  • "We know so much more than people did in those days, how can anything they said be true? Bronze aged Morons!"

Argumentum Ad Antiquitatem (Appeal to Tradition)

This fallacy is also known as proof from tradition, appeal to common practice (or belief), false induction, or the is ought fallacy. This is a common logical fallacy in which a thesis is deemed correct on the basis that it correlates with some past or present tradition. The appeal takes the form of "this is right because we've always done it this way."

An appeal to tradition essentially makes two assumptions:

    * The old way of thinking was proven correct when introduced. In actuality this may be false — the tradition might be entirely based on incorrect grounds.
    * The past justifications for the tradition are still valid at present. In cases where circumstances have changed, this assumption may be false.

The opposite of an appeal to tradition is an appeal to novelty (see above), claiming something is valid or correct because it is new, or the new way.

  • "Our society has always ridden horses. It would be foolish to start driving cars."
Rebuttal: we want to travel farther and horses are no longer adequate for traveling such great distances. Furthermore, there was a point in our past where our ancestors made the change from walking to riding horses.
  • "Your invention is a bad idea because it has no historical precedent."
Rebuttal: the fact that something has not been previously attempted does not guarantee it will fail. Moreover, there is a first time for everything.
  • "These rules were written 100 years ago and we have always followed them. Therefore, there is no need to change them."
Rebuttal: the society in which the rules were written has changed, and thus those rules may no longer be applicable.
  • "Murdering innocents is wrong, because it has been considered so since the dawn of civilization."
Rebuttal: the correctness of the conclusion is a matter of ethics, not logic, but the reasoning is fallacious. Murdering is considered to be wrong because of the (ethical) reasons given when it was first considered so (reasons which still hold), but not because of the tradition condemning it. Had it always been considered correct, we could still use those ethical reasons to start condemning it now.
  • "Men should always pay for dates, because men have always paid for dates."
Rebuttal: Until recently, men were the only sex that could earn substantial income and wealth. Consequently, they were the only sex that had the ability to pay for dates, and it became customary to do so. This however, is no longer the case. The financial disparity that once existed between the two sexes to justify such a tradition no longer exists today. Thus, this custom may no longer be applicable; and may be quite abusive given the current circumstances.
  • "Women should stay at home, because women have always stayed at home."
Rebuttal: Women stayed at home because of very limited alternative functions in society due to gender based education. However, this is no longer the case today. Therefore, it would be incorrect to derive the same conclusion based on history when the current circumstances are indeed different. Under modern economic conditions, a family might actually receive greater benefit from an additional household income than someone who simply stays home.

 (Correlation does not imply causation, Questioable Cause, False Cause)

  • cum hoc, ergo propter hoc
  • (Latin for "with this, therefore because of this")
  • non causa pro causa
  • ("non-cause for cause" in Latin)

This is a logical error I encounter frequently in discussions with Atheists in particular, who, in trying to demonstrate their world view, try to draw parallels between Christianity and other, sometimes older, religions or philosophies (Mithraism for example), in an attempt to marginalize Christianity.

Further, I have also frequently encountered this tactic from protestants who attempt to draw parallels between Catholicism and Paganism specifically.

See Also:



Recommended Reading

Principium Contradictionis (The Principal of Contradiction)

Reductio ad absurdum


This document may contain public domain content from Wikipedia. Some content may be based on context from Nizkor though in general I have done my best to remove context directly based on that page.