Sveinbjorn Thordarson

Úr The Economist, greininni The non-denial of the non-self:

IN THE 1940s a philosopher called Carl Hempel showed that by manipulating the logical statement “all ravens are black”, you could derive the equivalent “all non-black objects are non-ravens”. Such topsy-turvy transformations might seem reason enough to keep philosophers locked up safely on university campuses, where they cannot do too much damage. However, a number of computer scientists, led by Fernando Esponda of Yale University, are taking Hempel's notion as the germ of an eminently practical scheme. They are applying such negative representations to the problem of protecting sensitive data. The idea is to create a negative database. Instead of containing the information of interest, such a database would contain everything except that information.

The concept of a negative database took shape a couple of years ago, while Dr Esponda was working at the University of New Mexico with Paul Helman, another computer scientist, and Stephanie Forrest, an expert on modelling the human immune system. The important qualification concerns that word “everything”. In practice, that means everything in a particular set of things.

[...]

An interesting extension of the idea might be to use negative surveys to collect sensitive information privately. Dr Esponda gives the example of a negative survey in which respondents are asked to tick the box of one sexually transmitted disease they do not have. He reckons that this would be sufficient to estimate the population frequency of each disease, without having to ask people whether they actually suffer from such diseases—which is intrusive and also invites lying. As he puts it: “In Hindu philosophy, to find out who you are, you ask what are you not. Then you are left with what you are.”

Jahá, það er ekki oft sem það er minnst á Carl Hempel í mainstream fjölmiðlum. Reyndar er hin svokallaða "hrafnaþversögn" Hempels mjög skemmtileg, og tengist tilleiðslu. Því oftar sem við sem skynjum að e-ð af einhverri tegund hefur e-a ákveðna eiginleika, því meiri líkur teljum við á því að framtíðar eintök af viðkomandi hafi þessa sömu eiginleika.

Ef við hugsum okkur kenninguna "Allir hrafnar eru svartir", þá getum við farið og skoðað fjöldann allan af hröfnum. Í hvert skipti sem við sjáum svartan hrafn, færast stoðir undir kenningu okkar. En nú kemur vandinn -- því kenningin "Allir hrafnar eru svartir" er röklega jafngild kenningunni "Allt sem er ekki svart er ekki hrafn". En það skiptir engu máli hvað í kringum okkur við skoðum -- ef það er ekki svart og ekki hrafn, þá rennur það stoðum undir kenninguna "Allt sem er ekki svart er ekki hrafn", og þ.a.l. röklega einnig kenninguna "Allir hrafnar eru svartir".