Technology changes the world. It changes what we know, and it changes our language.

People talk about "domain knowledge" as something to model, something definite and pretty much fixed. Yet it always turns out to be anything but: it's just the design of the larger system that contains whatever the project concerned happens to be working on.

For example, "aircraft" in the 1930s meant propeller-driven flying machines with a monocoque construction, i.e. a fuselage fixed to a single hollow wing with an aluminium skin, as opposed to a biplane made of wood and stressed wires and canvas. But already in the late 1940s "aircraft" could also mean a jet fighter plane; and by the 1950s it could mean a jet airliner, or for that matter a helicopter.

turns out to be wrong sooner or later, and generally sooner. About the only part of the sentence we can still assent to is the bit about flying machines, and that sounds distinctly old-fashioned, even without the musical echoes of "those magnificent men in their flying machines". Words, like all other symbols, mean whatever we want them to mean, as the mathematician Lewis Carroll took pleasure in reminding us, in the figure of Alice (in Wonderland, and Through the Looking Glass).

Domain models, like all other models, are contingent on reality: they are helpful if (and only if, as a logician might say) they correspond usefully to the world.

The problems of tacit knowledge, the transitoriness of domain knowledge, the difficulty of validating requirements: they are all here.

I began by saying that technology changes the world. This reflection came to me very simply as I learned to use a USB audio capture device, a tiny thing on a cable that plugged clumsily into the fat phono jack on my hi-fi amplifier, so that I could listen to my antique "record" collection - now there's a domain term that has changed its meaning.

In the old days - from the 1960s to the 1980s - a record player (or "deck") was a mechanical device which spun a naked disk of fragile plastic or "vinyl". The record formed the heart of an "album", which consisted of a "sleeve" of white paper to keep dust from the delicate recording surface, one or more records, and a cover made of thin card, often extravagantly decorated. In those far-off times, sound was recorded directly and mechanically, as wobbles in a fragile groove on a disk. The disk was played back by a diamond needle resting in the groove, being made to vibrate by the wobbles in the groove. The tiny vibrations were passed as a feeble electrical signal to an amplifier, and enlarged to be loud enough to hear by analogue electronics. Not surprisingly, dust, vibration and any kind of mechanical disturbance were fatal to the enterprise, as was leaving the "long player" ("LP") in the sunshine or sitting on a hot radiator or simply leaning over with any sort of pressure on it, all of which caused disastrous and irreversible warping. All of these domain facts were well known to every music-lover, and scrupulously ignored by all the record companies until an alternative - the CD - appeared. They were the highest fidelity recording media of their day.

And among that antique collection of LPs was an extraordinary recording: a suite of "songs", written by none other than Ludwig Wittgenstein himself, and performed by M. A. Numminen, "a Finnish writer and musician of a determinedly 'underground' persuasion", as the "liner notes" say - ah, another piece of long-gone domain knowledge: in those distant days, recordings came with some words of careful, humorous, serious, esoteric or just plain banal explanation, often accompanied by photographs, drawings, even holograms; and the lyrics of songs or "tracks" - they were visible as silky circular bands on the record's surface - were generally printed, large enough to read, on the liner itself.

A detail from "The Tractatus Suite", 1989, lyrics by Wittgenstein, performed by M.A. Numminen's band

In this case the lyrics occupied the whole of the inside of the album cover - about 2 square feet (60cm x 30cm), as the album folded out to reveal the words in German, English, French, Swedish and Esperanto. You can read them for yourself in Tractatus Logico-Philosophicus, of course. But the front and rear covers are harder to find, so I have scanned and linked them here.