"Every program attempts to expand until it can read mail. Those programs which cannot so expand are replaced by ones which can." Coined by Jamie Zawinski (who called it the "Law of Software Envelopment") to express his belief that all truly useful programs experience pressure to evolve into toolkits and application platforms (the mailer thing, he says, is just a side effect of that). It is commonly cited, though with widely varying degrees of accuracy.
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