From a neat NYRB post about the philosophical underpinnings of word processing applications:
No writer has ever thought about the exact percentage of italics in a line of type, but Word is reduced to this kind of arbitrary principle because its Platonic model—like all Platonic models—is magnificent in its inner coherence but mostly irrelevant to the real world. In order to make a connection between heavenly ideas and tangible realities, Plato himself was reduced to inventing something he called the Demiurge, an intermediate being who translates the ideal forms in heaven into something tangible in the world. The Demiurge is an early instance of what programmers call a kludge—a clumsy and illogical expedient for dealing with a problem that seems too intractable to solve more elegantly. Word’s 50-percent rule for applying styles is a descendent of the Demiurge, and just as much of a kludge.
The inventors of WordPerfect had no grand ideas about the form of a document. Instead they looked over typists’ shoulders and tried to find ways of imitating their actions on a computer keyboard. So, when you want to change the margin in WordPerfect, you press a few keys to perform the computer equivalent of pushing the lever on a typewriter. You change the margin, and then, later, you might change it back again. Word’s intellectual model is effectively timeless: you paint the text with its attributes. WordPerfect’s is active and progressive: you change a setting, continue typing, and then change some other setting. Auden’s word “mediocrity” seems too strong to apply to WordPerfect, as it was too strong to apply to Isocrates or John Dewey, both of whom had something very like genius in their clear-sighted, unprejudiced perception of the world as it is.
On a very practical level, I now have a better idea of the cause of bizarre Word behaviors like “If I change the font of a single entry in a bulleted list, it also changes the font of the last character in the previous line.”
More interestingly, however, is how this discussion illuminates the ways in which the assumptions that go into the design of a tool directly impact the ways in which people use that tool, to say nothing of how they shape the output of the tool.