> You always have your mind.

Yeah. Human mind is very limited tool without external support. That's why writing was such a boon for humans and notation was such a boon for math.

> Are you incapable of just sitting and thinking about things?

Yeah. I can but that rarely acomplishes anything. If you think you can solve a problem just with your mind think again. Most problems are problems because you lack some information. And you won't get any missing information just by thinking. And if you have all the information, you just need to process it then you are still out of luck. Even moderately complex processing let's say roughly as hard as derivating moderately simple calculus requires at least a pen and piece of paper to support you wonderful mind with crappy operational memory that can hold at most seven things at once.

I would much rather do the thinking with my phone than without it.

I'm not Feynman and Einstein had plenty of paper.

You can't daydream if you must focus your attention on something boring. I wonder if Einstein would have come up with Relativity if he had to write mind-numbing reports all day.