"You are not supposed to..." in general I agree that we should take first principles based approach always as an option - from point of view of taking intellect

"You are not supposed to..." in general I agree that we should take first principles based approach always as an option - from point of view of taking intellectual pride in ones work. However, a large category of real world problems is solvable only through a combination of solutions to hard problems. If I have to solve three hard problems before I can have a complete solution is at hand - and by hard I mean months of work - my memory is so poor that the current value of a solved problem does not depend on whether I or someone elso solved it. I've forgotten the details anyway - and the only thing I have are the implementation and documentation. I have a limited lifespan but the number of interesting problems is infinite. I can't really find a calculus that says it's far more valuable to solve the known problems several times rather than new interesting problems. Because that's what you are implying (with my memory) - if never using existing solutions is preferable I would never get anything done. The same applies for most of civilizations activities. Don't confuse practicality as the opposite of love of knowledge.