A difference that does not make a difference is not a difference at all.
-- Anon. Pragmatist's Motto

Nothing is more practical than a good theory.
-- Ludwig Boltzmann 

...I can't define my wife, but I can recognize her.
--E. Atlee Jackson, as quoted by John Horgan, The End of Science

Everything that's worth understanding about a complex system, can be understood in terms of how it processes information.
-- "Lloyd's Hypothesis", named after Seth Lloyd, Moore's Law and the Ultimate Laptop

Emergent behavior is that which cannot be predicted through analysis at any level simpler than that of the system as a whole... Emergent behavior, by definition, is what's left after everything else has been explained.
-- George B. Dyson