Intelligence can be practiced.
Clearer thinking is a discipline rather than a birthright or a rank, which means anyone willing to put in the practice can get better at it.
Nobody thinks it is strange that a runner runs every day or that a pianist plays scales. Thinking is the one skill we treat as fixed, as if the brain you were issued is the brain you get and the only question is how it measures against everyone else's.
The claim here is that thinking works like everything else and responds to use. The practice is easy to describe and easy to skip: every day, make your brain do some work it would rather not do. Read something a little too hard for you. Work a problem by hand before you look up the answer. Explain something you think you understand to someone who will ask the next question. Learn a game you are bad at.
The subject matters less than the strain. A crossword, a language, a proof, the wiring diagram for the thing you are fixing: any of these will do as long as it is slightly beyond you. The moment it gets comfortable, it has stopped being practice.
The other half of the claim is the word anyone. Clear thinking is a habit rather than a rank, and a habit belongs to whoever keeps it. Pick one thing and do it today, then do it again tomorrow, which is the part that matters.
IV. Tools should enlarge us.