What is Extensional Pruning?

We are guilty of extensional pruning if we use words in their commonly accepted meaning, but retreat when challenged into a strictly literal definition. The fallacy becomes possible because there are two ways of understanding what words mean. We can describe the properties of what we refer to, or we can give examples. The first is called the ‘intension’, and the second is the ‘extension’ of the word. We could convey the sense of an expression such as ‘movie star’, for example, either by describing the role of lead actors and actresses in films, or by listing several well-known stars.

Words carry nuances of meaning by their associations. Little tendrils of thought ripple around them, evoking all kinds of ideas dependent on past associations. These nuances are part of the meaning of the word, provided they are understood by user and hearer alike. The fallacy of extensional pruning takes place when the user subsequently retreats from that meaning by insisting upon only a literal ‘intentional’ definition.


While I said I would accept an inquiry, I at no time said that it would be independent, that it would be a public one, or that its findings would be published.

(He might be correct in a limited, technical definition of the word. But this is not what most people normally understand from the associations they make with previous inquiries.)


The fallacy is committed by saying one thing, but permitting another to be understood. A contention must be the same to

both user and hearer, or no reasoned discussion is possible. There are two ways of committing this fallacy: one is to mislead at the outset, the other is to retreat to a restricted definition in order to escape weaknesses in the position.


All we said was that we’d install a switchboard. We didn’t say it would work.

(Nor did they.)


Advertisers often take pruning shears over the extravagant claims they have made.


We’ll take your one-year-old car as trade-in, at whatever you paid for it.

(Strictly speaking, you paid one sum for the car, and another sum for the tax. They are not offering to give you the tax back as well, whatever you might have thought.)


Friends who are free with advice often cut back the meaning in a similar way, after the consequences have emerged.


Look, I know I said you’d feel like a millionaire. I know lots of millionaires who feel pretty miserable. Stop complaining.

(You would feel like a swine if you hit him, but you probably know lots of swine who’d enjoy it.)


The extensional pruner announces his activity. Like the bow wave of an advancing ship, his utterances mark his passage. The inevitable ‘all I said was…’ and ‘if you examine my exact words…’ show him to be a man of great qualifications. You recognize him as the man who never really said at all what most people took him to be saying. The fine print one always watches for is in this case in the dictionary.

You can add extensional pruning to your repertoire once you are adept at making a limited statement pass itself off as a wider one. Gather yourself a collection of phrases whose meaning is understood by everyone, even though the words themselves are more restricted.


I said I’d get you another drink if I was wrong: water is another drink.

I said I wouldn’t have any more cigarettes until later in the week. Five minutes afterwards was later in the week.

(Speak softly, and carry a big  dictionary.)


The above article is from the book How To Win Every Argument by Madsen Pirie. The article is only for educational and informative purposes to explain and understand formal logic and logical fallacies. It is a great book, definitely worth a read!