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Immutability: The true constant is change. Mutation hides change. Hidden change manifests chaos. Therefore, the wise embrace history.
If you have a dollar, and I give you another dollar, it does not change the fact that a moment ago you only had one dollar, and now you have two. Mutation attempts to erase history, but history cannot be truly erased. When the past is not preserved, it will come back to haunt you, manifesting as bugs in the program.
Separation: Logic is thought. Effects are action. Therefore the wise think before acting, and act only when the thinking is done.
If you try to perform effects and logic at the same time, you may create hidden side effects which cause bugs in the logic. Keep functions small. Do one thing at a time, and do it well.