Fallacies of Presumption
Fallacies of presumption are based on unjustified or problematic assumptions.
1. False Dichotomy (False Dilemma)
In this fallacy, only two alternatives are presented, even though there are in fact more options.
Example: "Either you support this measure, or you are against progress."
Why is this flawed? There may be other positions, e.g. one could be in favour of progress but prefer different measures.
2. Loaded Question
A loaded question contains a hidden assumption that the respondent implicitly accepts through any direct answer.
Example: "Have you stopped evading your taxes?"
Why is this flawed? Regardless of whether the answer is "yes" or "no", the respondent implicitly accepts the assumption that they evaded taxes in the past.
3. Petitio Principii (Circular Reasoning, Begging the Question)
In this fallacy, the conclusion is already presupposed in the premises.
Example: "The Bible is the word of God, because it says of itself that it is inspired by God."
Why is this flawed? The claim that the Bible is the word of God is justified by referring to the Bible itself, which is circular.
4. Post Hoc Ergo Propter Hoc (After This, Therefore Because of This)
This fallacy assumes that an event which occurs after another event was caused by it.
Example: "I wore a black hat, and then it rained. So black hats cause rain."
Why is this flawed? The temporal sequence alone does not prove causation. It could be a coincidence, or other factors could be at play.
5. Cum Hoc Ergo Propter Hoc (With This, Therefore Because of This)
Similar to Post Hoc, but here causation is inferred from a correlation.
Example: "In countries with high chocolate consumption there are more Nobel laureates. So chocolate makes you more intelligent."
Why is this flawed? Correlation does not imply causation. There could be other factors that influence both chocolate consumption and the number of Nobel laureates (e.g. prosperity, the education system).
6. Slippery Slope
This fallacy claims that a relatively small first step will inevitably lead to a chain of events ending in a catastrophic outcome.
Example: "If we legalise cannabis, soon all hard drugs will be legal, and society will descend into chaos."
Why is this flawed? It is assumed, without sufficient justification, that one step will necessarily lead to a whole chain of events. The causal connection between the individual steps is not demonstrated.
7. Argumentum ad Ignorantiam (Appeal to Ignorance)
This fallacy claims that something must be true because it has not been proven false, or vice versa.
Example: "No one has proven that ghosts do not exist. So they must exist."
Why is this flawed? The absence of evidence against a claim is not evidence for the claim. The burden of proof lies with the person making a positive claim.