Trees sprout up just about everywhere in computer science... - Donald Knuth

In fact, my main conclusion after spending ten years of my life working on the T_EX project is that software is hard. It’s harder than anything else I’ve ever had to do.

I am assuming that God exists and I am glad that there is no way to prove this. [Because] I would run through the proof once, and then I'd forget it, and I would never speculate about spiritual things and mysteries otherwise. And, I think, my life would be very incomplete.

If you find that you're spending almost all your time on theory, start turning some attention to practical things; it will improve your theories. If you find that you're spending almost all your time on practice, start turning some attention to theoretical things; it will improve your practice.