Have you ever thought too deeply about something? I most certainly have. Doing so can drive you mad. Sometimes you have to learn to step away from things before you can make any real progress. As Nikola Tesla said, thinking deeply and thinking clearly are definitely not the same thing. I could also bring up Albert Einstein’s definition of insanity: trying the same thing over and over and expecting a different result.
Look at how many storybook villains work so hard but forever fruitlessly towards an impossible goal. You can't blame them for not thinking deeply and being persistent. But clarity of thought certainly isn't in their repertoire. It's tragic, really. Personally I prefer villains who can in fact think clearly. Those tend to be the most dangerous and fascinating ones for me. Exercises in futility get tiresome for me after awhile.
Sadly enough, real life people exercise futility far too often. I most certainly am guilty of exercising insane levels of futility. If you start thinking too deeply about something that can unfortunately mess you up fairly badly. If you start finding your thinking derailing, take a break, no matter how urgent something might be. You're better off finding a moment of clarity than sinking into hours of deep thought that could just send you down a deep dark tunnel of insanity.
Back when Tesla first said this quote, he felt that the scientists of his day were thinking more deeply than clearly. With how many “experts” and highly specialized fields we have today, it’s easier than ever to think a bit too deeply about something. It’s easy when studying something too closely to lose perspective. Clear thinking is the best way to go. If you find yourself lost in thought, then perhaps you’ve been on that track a bit too long.