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There are many complicated problems in the world which have no simple solution. One good example is traffic congestion in cities - if you widen the roads, this attracts more drivers, often making the situation worse. It also makes the area more dangerous for pedestrians and cyclists, encouraging them to use cars instead. Even if the traffic ends up moving faster, pollution and associated health risks are often increased because there is more traffic.
A problem situation (like traffic in cities) is said to be Pareto Optimal if you can't make any change at all without making the situation worse in some important respect.
Conversely, a Pareto Optimisation is a solution which, when it is applied, leaves nobody any worse off.
The Pareto Conjecture is the theory that the real world does not contain any Pareto Optimisations.