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A false analogy is an unjustified inference drawn on the basis of similarities between two items or types of items. The justification of an inference based on analogical reasoning depends on the number and strength of known similarities and dissimilarities of the items being compared. If there are very few known similarities or if there are a few known very great dissimilarities, then drawing inferences based on the comparison is unjustified. The result is a false analogy.
For example, Dean Radin (1997, p. 33) compares a participant in a psi experiment to Mickey Mantle, a baseball player, and argues that since getting a hit one out of three times is considered very good for a batter, it ought to be considered very good for a psychic. Gary Schwartz (2002, p. 53) makes a similar argument (though he calls it "a metaphor") comparing Michael Jordan, a basketball player, to a psychic. There are too many significant differences between hitting a baseball or making a basket and getting a psychic "hit" in a telepathy experiment or in a test of mediumship to justify this conclusion. So, the argument is a false analogy. It is also very misleading because there is no proof that guessing cards or pictures—the typical task in an ESP experiment—or having one's words validated by a subject in a mediumship test are skills. We know that hitting a baseball or making a basket is a skill and that is what allows us to measure a player's performance by comparing it to the performance of other players. Parapsychologists measure psychic performance against a statistical probability of chance, which shares nothing in common with the statistical average of all baseball players or basketball players except that they're both statistics. And Schwartz measures psychic performance against either no standard at all or against a calculated "conditional probability." Again, these have nothing in common with the performances of athletes.
It is true that cold reading is a skill but it is a skill whose success depends on a subject validating the words of the reader. It does seem that Schwartz is actually measuring cold reading when he tests mediumsdespite his claim to the contrarybut for the "hits" of a cold reading to be analogous to those of a basketball player making baskets we would have to let the player's most avid fans determine what counts as making a basket. Hitting the rim or the backboard, or even shooting the ball over the backboard into the rafters might be counted as a hit by some fans.
Had Radin or Schwartz compared psychic hit rates with major league baseball fielding averages instead of batting averages or percentage of shots made in basketball, the result would not look so favorable, as even the poorest fielders will be successful 95% to 98% of the time. The worst team ever was successful at fielding 94% of the time.*
more false analogies
To support a belief in the homeopathic law of similars (like cures like), Dana Ullman, an advisory board member of alternative-medicine institutes at Harvard's and Columbia's schools of medicine, argues that the human body reacts to medicines the way a piano string reacts to the vibrating string of another piano:
If one piano is at one end of a room and if one strikes the C key, the C notes in another piano in the same room will reverberate. This experiment works because each key is hypersensitive to vibrations in its own key. This is called 'resonance.'
It is also called a false analogy. The human body shares almost nothing in common with a piano string, and reverberation is unlike anything in the body's natural healing system. The analogy may make sense to Ullman because of acceptance of the notion that homeopathy is a type of "energy medicine." Disease is caused by blockage of energy and health is restored when the energy flows freely.
Ullman provides another false analogy by comparing the infinitesimal amounts—sometimes equaling zero—of homeopathic substances to tiny atoms containing vast amounts of energy.
There are many phenomena in nature in which extremely small doses of something can create powerful, even very powerful, effects....One certainly cannot say that the atomic bomb is a placebo just because some extremely small atoms bump into each other.
From this he concludes that we shouldn't dismiss the successes of homeopathy to the placebo effect. It is not relevant to compare the energy of no molecules (which is often what remains of the "active" substance in a homeopathic remedy) to the energy in atoms. If there are any molecules of the "active" substance left after dilution, they have as much energy as their atoms possess, no more and no less.
Here's a false analogy from Jerry Falwell:
Just as no person may scream ‘Fire!’ in a crowded theater when there is no fire, and find cover under the First Amendment, likewise, no sleazy merchant like Larry Flynt [publisher of Hustler magazine which had been sued by Falwell] should be able use the First Amendment as an excuse for maliciously and dishonestly attacking public figures, as he has so often done.
There are so few similarities between yelling Fire! in a crowded theater and writing "malicious stuff" in a pornographic magazine that it is pointless to evaluate this kind of comparison in any detail. (This reasoning also begs the question. It assumes that the "malicious writings" are of "such a nature as to create a clear and present danger that they will bring about the substantive evils that Congress has a right to prevent.")
even good analogies sometimes fail
It is generally accepted among scientists that rodents, rabbits, monkeys, and other mammals share enough significant biological features to justify, for example, testing drugs on animals before trying them on humans. (We're talking about logical justification, here, not ethical justification, which is a different matter altogether.) For example, Frederick Banting and Charles Best discovered insulin by working on dogs.* Millions of human lives have been saved by their work on dogs. However, not all work with animals has had such a happy ending. The experimental drug TGN1412—designed to treat rheumatoid arthritis, leukemia, and multiple sclerosis—had been given to 20 monkeys in tests before it was given to six humans in a clinical trial, causing all of the humans to go into almost immediate organ failure. None of the monkeys suffered organ failure. The drug apparently over-stimulated the human immune system "sending white blood cells called T cells rampaging through the body destroying its own tissues."* Why was there such a different reaction in humans? Obviously, there is some significant difference between humans and monkeys that relates to the immune system.
It's believed differences between a cell-signaling protein in humans and animals may explain the unexpectedly severe reaction in the previously healthy young men....*
TGN1412 is designed to bind to a particular immune system protein called CD28. Dr. David Glover, an expert on antibody treatments, suggests that the protein the drug targets may not be the same in all species. That fact, if it is a fact, can only be discovered empirically, and it illustrates one of the main limitations of analogical reasoning that draws conclusions about one species based on its similarities to another species.
(The TGN1412 episode also illustrates another important point, as noted by Abel PharmBoy (3/21/06): many concoctions hawked by complementary and integrative medicine quacks are recommended because they will "boost your immune system." If they actually did, the result might be painful or catastrophic.)
Analogical reasoning is a type of inductive reasoning and its conclusions follow with some degree of probability, not with necessity. Generally speaking, the more similarities things are known to share in common, the more likely they will share in common some further feature. If you've bought ten new Hondas over the past ten years and they've all been great cars, you are justified in being confident that the new Honda you are purchasing will probably be a great car. But, of course, you could get a lemon.
See also argument from design.