Welcome & welcome back to this series on logic! I've been posting these diaries on Mondays, Wednesdays, & Fridays in hopes of creating a resource to aid in discussions at Daily Kos & elsewhere in life.
The series so far:
Proposing A Series On Logic
Informal Fallacies, Part 1 — Ambiguity
Informal Fallacies, Part 2 — Getting Personal
Informal Fallacies, Part 3 — Un-Appealing Gimmicks
Informal Fallacies, Part 4 — Generally Speaking
Informal Fallacies, Part 5 — "That Begs The Question"
Some more important relevance fallacies beyond the Kos Divider...
Argument From Ignorance (Argumentum ad Ignorantium).
Someone argues that something is true because nobody has proven it false, or else that something is false because nobody has proven it true. Unless, as in a court case, the evidence itself is the issue, lack of pre-existing proof & essential truth value are separate issues. Spiritualist "mediums", creationists, & others who like to play with radical re-interpretations of reality often fall back on some form of this when the inevitable question of evidence finally arrives at their doorsteps — & the best response in such cases usually consists of insisting that the bringer of a claim must also bring the evidence for it.
False Cause.
The human race has made errors in determining the proper cause-&-effect relations between events for so long that some philosophers such as Frederich Wilhelm Nietzsche have considered this the most fundamental error in all of logic. At any rate, among other mistakes of this sort, one of the most fundamental takes the shape after therefore because (Latin post hoc ergo propter hoc). This frequently goes along with forms of the "placebo effect", as people, being pattern-matchers, often link something that happened before an event to something that happened afterward simply because it happened 1st, whether or not the facts actually relate the events to each other (& people who have already been convinced by such thinking into such notions as the full moon or solar eclipses being bringers of doom can be hard to convince otherwise).
(Hmm, these fallacy lists have begun to look more like lists of rhetorical devices lately. I wonder where this will have led by the end of the week...)
The floor is now open to questions & comments.