Ignore sunk costs - decisions made yesterday are not relevant to the future

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When making a new decision, we must ignore what we acquired yesterday.
The skills we earned, the machines we purchased, the privileges we were given—they are all gifts to you from the you of yesterday. Like all gifts, they’re optional. You don’t need to accept them or keep them around.
It’s one of the most profound and difficult lessons every MBA is taught: Ignore sunk costs. Money and effort you spent yesterday should have nothing to do with decisions you make tomorrow, because each decision is a new one.