See: How to Make a Complete Map of Every Thought you Think [pdf]

An excerpt from the introduction (tldr: beware of eating yourself):

This book is about how to make a complete map of everything you think for as long as you like.

Whether that's good or not, I don't know- keeping a map of all your thoughts has a freezing effect on the mind. It takes a lot of (albeit pleasurable) work, but produces nothing but sight.

If you do the things described in this book, you will be immobilized for the duration of your commitment.The immobilization will come on gradually, but steadily. In the end, you will be incapable of going somewhere without your cache of notes, and will always want a pen and paper w/ you. When you do not have pen and paper, you will rely on complex memory pegging devices, described in "The Memory Book". You will never be without record, and you will always record.

You may also articulate. Your thoughts will be clearer to you than they have ever been before. You will see things you have never seen before. When someone shows you one corner, you'll have the other 3 in mind. This is both good and bad. It means you will have the right information at the right time in the right place. It also means you may have trouble shutting up. Your mileage may vary.

I think you'll get more concrete suggestions if you explained what you hope to accomplish with this proposed task.

I've settled for (1) keeping a structured list of all books from which I've learned something worthwhile, and (2) a log for current ideas (with no more than a few short entries a week). This is sufficient to locate and efficiently relearn most barely-remembered ideas when they become relevant again.

Writing down everything you know seems pretty pointless. Writing down everything you fear forgetting might give you a smaller but more useful list, since it lets you cull out anything you're in no danger of forgetting (e.g. all the arithmetic facts) as well as anything you wouldn't care if you forgot (e.g. the vast majority of knowledge in your head).

I actually sort of do this, in a private git repository where (alongside lists of interesting typically-paragraph-sized quotes) I keep lists of interesting (typically-sentence-fragment-sized) topic names. Sometimes the names serve as mnemonics that merely remind me of an interesting fact I once encountered but haven't thought about recently (e.g. "Rai stones"). Sometimes I'll skim through the lists and encounter a topic that I've completely forgotten about (e.g. "burying the corpse effect") and I'll quickly Google to see why I once thought it was so interesting to begin with. A little organization helps. E.g. "burying the corpse effect" was in my "economicsbits" file under the hierarchy "Financial markets, investment", "Market manipulation", "Cornering the market", so it was easy to tailor web searches to lead me to results from economists rather than morticians.

I don't know if this is useful for anything more than entertainment. TimS had a very good question here.

I started something like this awhile ago. I was trying to write papers for one of my classes and couldn't find a reference I needed. After about the third time this happened, I figured I ought to make some kind of searchable list of references with summaries about what they contain, and links to the file. I use a google document now, with summaries of books I've read and notes from my classes, in addition to references. What I really want is something like workflowy where I can collapse bulleted points. Workflowy would be fine, but I'd be worrying about going over their limit and having to pay for it, since I have a lot of bullet points. In the meantime, I use google docs' "table of contents" feature so I have that orderly list I want.

I don't put "everything" in it. My general rule is that it has to be either useful, something I'd likely forget, or something interesting. I also link to everything so I don't have to search my history.

What do you mean by everything? Surely not literally everything?

The value/work ratio seems pretty low to me. Is this going to help you achieve your goals? If so, how? If not, is it fun enough to be worth it for that reason?

Writing down only every arithmetic fact you know, assuming you have basic knowledge of math, would, in theory, take an infinite amount of time. In practice, the universe would end first.