"If you want an application to be portable, you don't necessarily create an abstraction layer like a microkernel so much as you program intelligently."

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Suppose that you want to teach the 'cat' concept to a very young child. Do you explain that a cat is a relatively small, primarily carnivorous mammal with retractible claws, a distinctive sonic output, etc.? I'll bet not. You probably show the kid a lot of different cats, saying 'kitty' each time, until it gets the idea. To put it more generally, generalizations are best made by abstraction from experience.

— Ralph P. Boas, Jr.