Storing too much information will be a hassle when reading through it later, storing too little will have little benefit as well.
Some criteria that we can use to know what is noteworthy:
- Is it useful to me in some way?
- Does it inspire me?
- Is it something personal?
- Does it surprise me?

All in all they basically boil down to: does it resonate with me?

We should listen to our intuition about this rather than being to analytical. The latter will make it a less pleasant experience; don't feel like having to check off a checklist before allowing something in the second brain. Just listen to your intuition. 

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Building a second brain - Chapter 4