Jim Rohn compares protecting your mind to guarding what goes in your coffee: sugar is fine, strychnine kills you regardless of who puts it there. You live with whatever you let into your mental factory, so guard it carefully.
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I said, "What would happen if somebody dropped sugar in my coffee?" They said, "Well, you'd be okay." I said, "What if somebody dropped strychnine in my coffee?" They said, "Well, you'd be dead." I said, "Correct." I said, "What if my worst enemy drops in the sugar?" They said, "Will you be okay?" I said, "What if my best friend, even by accident, drops in the strychnine?" They said, "Well, you'd be dead." I said, "Correct." See, it doesn't matter who hands you the bad stuff. Every day, stand guard at the door of your mind. How important? You decide what goes into your mental factory. Don't let anybody just dump anything they want to in your mental factory, because you've got to live with the results.
One of the most intelligent dudes of all time. I love watching this stuff. Simple concepts that, that hold true for me too.