Knowledge that is familiar is background knowledge.
A predictive theory that has only background knowledge is
a 'rule of thumb'. Rules of thumb are illusions.
If a rule of thumb happens to work in its prediction there is always
an explanation, denying the explanation is like believing in the
supernatural.
On the other hand, there is always an explanation why a rule of thumb
fails. For rules of thumb are always narrowly restricted in scope and
outlook and thereby hold only in a narrow range of familiar circumstances.
Adapted from source: The Beginning of Infinity by David Deutsch
Books by Peter Cheevers at:
http://www.amazon.co.uk/s/ref=nb_sb_noss?url=search-alias%3Ddigital-text&field-keywords=peter+cheevers&x=0&y=0
A predictive theory that has only background knowledge is
a 'rule of thumb'. Rules of thumb are illusions.
If a rule of thumb happens to work in its prediction there is always
an explanation, denying the explanation is like believing in the
supernatural.
On the other hand, there is always an explanation why a rule of thumb
fails. For rules of thumb are always narrowly restricted in scope and
outlook and thereby hold only in a narrow range of familiar circumstances.
Adapted from source: The Beginning of Infinity by David Deutsch
Note: The term ' rule of thumb' is thought to originate with wood workers who used the width of
their thumbs (i.e. inches) rather than rulers for measuring things, cementing its modern use as an
imprecise yet reliable and convenient standard. This sense of thumb as a unit of measure also
appears in Dutch in which the word for thumb, duim, also means inch. The use of a single word or cognate for "inch" and "thumb" is common in many other Indo-European languages, for example, French: pouceinch/thumb; Italian: pollice inch/thumb; Spanish: pulgada inch, pulgar thumb; Portuguese: polegada inch, polegar thumb; Swedish: tum inch, tumme thumb;Sanskrit: angulam inch, anguli finger; Slovak: palec, Slovene: palec inch/thumb, Czech: palec inch/thumb.
their thumbs (i.e. inches) rather than rulers for measuring things, cementing its modern use as an
imprecise yet reliable and convenient standard. This sense of thumb as a unit of measure also
appears in Dutch in which the word for thumb, duim, also means inch. The use of a single word or cognate for "inch" and "thumb" is common in many other Indo-European languages, for example, French: pouceinch/thumb; Italian: pollice inch/thumb; Spanish: pulgada inch, pulgar thumb; Portuguese: polegada inch, polegar thumb; Swedish: tum inch, tumme thumb;Sanskrit: angulam inch, anguli finger; Slovak: palec, Slovene: palec inch/thumb, Czech: palec inch/thumb.
Another possible origin of the phrase comes from measurement, in particular in agricultural fields. The plants need a fairly precise depth to seed properly, whether planted from seed or being replanted, but the depth can sometimes be estimated using the thumb. That is, a rule "(measurement) of thumb." According to Gary Martin, "The origin of the phrase remains unknown. It is likely that it refers to one of the numerous ways that thumbs have been used to estimate things—judging the alignment or distance of an object by holding the thumb in one's eye-line, the temperature of brews of beer, measurement of an inch from the joint to the nail to the tip, or across the thumb, etc. The phrase joins the whole nine yards as one that probably derives from some form of measurement but which is unlikely ever to be definitively pinned down."[6]
Books by Peter Cheevers at:
http://www.amazon.co.uk/s/ref=nb_sb_noss?url=search-alias%3Ddigital-text&field-keywords=peter+cheevers&x=0&y=0
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