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Logic and rhetoric
Handwaving is the act of glossing over a difficult or superfluous component in an argument, by ignoring or distracting from it, and hence treating it as a black box for the purpose of a discussion. It is a metaphor derived from the sleight-of-hand of a conjuror who relies on flamboyant hand movements to distract the audience.
The assumption is that the component in question is either outside the scope of the current discussion or simply doesn't need to be understood by the listener; as a result, while there may be legitimate reasons not to bother with the handwaved topic, handwaving can sometimes be used as a form of misdirection or outright bullshit in order to advance a faulty argument. The term is used in mathematical contexts where important steps are left out so that something is assumed to be true without any rigorous demonstration, which can lead to things being assumed to be true that are in fact unproven and false. In literary or film criticism it refers to a technique where an author ignores a plot hole and hopes the audience won't notice or tries to get around a plot hole by distraction.
What we are really interested in...
Woo-defender: Magnet therapy has been proven to work in so many cases.
Scientist: Well-designed clinical testing has shown no correlation between the use of magnetic therapy and any improvement in health.
Woo-defender: Clinical trials may be fun to chat about but we are talking about the thousands of real-life successes here. There are millions of them. It is the billions of real-life cases of success with magnetic therapy in real-life magneto-medicine that we are interested in.
"Just a theory" gambit
Scientist: That's not how evolution works. Evolution is based on natural selection from traits over time. As humans, we don't have any sort of gene that allows us to fly, since we don't need to. We can't even grow our own wings at will. It's not genetic at all. Unless a flood covers the Earth and we're forced to live above ground, then maybe, over hundreds or thousands of generations of natural selection, we may sprout wings.
Skinner’s Constant (also known as Flannegan's Finagling Factor) denotes:
“”That quantity which, when multiplied by, divided into, added to, or subtracted from the answer you got, gives you the answer you should have gotten.
There's a great many ideas that would work, if just one important item wasn't completely unknown. For such things we have the Skinner constant — the mathematical equivalent of a handwave.
Left as an exercise
Usually, the term "left as an exercise" (originated in and largely used in mathematical and computer science circles) implies that part of a proof or algorithm is sufficiently elementary as to be either obvious or readily worked out by the reader or student. However, it can be used in two less-than-sensible ways:
- As a potholed introduction to an exceptionally difficult or unsolved problem
- As part of an argument by intimidation