Humm--- I've used Personal Brain as my primary digital information organizer since version 1, and what I've found that works best for me is: keep every brain focused on what it does.
Now, you might ask why--- and the answer is the "holy grail" of information keepers---- SO I CAN FIND THINGS IN THEM MANY YEARS LATER!
With a "brain with a purpose", it only contains items that are useful to it. I have a general work brain, that contains a listing of all the common tasks I do for work. It lists my major work projects--- but those projects major details? That's all contained in their own brains. I do have a few duplicates because of this (for example, my "shortcut" thought to my SQL+ tool occurs many times, as it is in my current work brain, as well as each project brain that I needed to use it), but that is acceptable to keep my brains focused. When I am searching for the emails "Kristy Smith" sent me related to the requirements for the Constellation Action Tracker project, I go into my Constellation Action Tracker project brain and do a big search for her name. I'll get only the "signficant" emails she's sent me about it, (as well as any work order and trackers with her mentioned). If I had did that in a "One Brain for everthing", I'd get ALL her "somewhat importish seeming" emails and mentions--- and at that point, I might as well let my email app do the searching--- as they are going to be very similar results, only the email app will ONLY have the emails in it--- and not all the various OTHER documents where she is mentioned or originated.
When I have to go back into a brain that I haven't used in NINE YEARS--- it is CRITICALLY important that it only have the pertanent information contained in it. I don't need the intervening nine years of general information detrius that accumulates over the course of a career (or life, for that matter).
Frankly, I am not missing the gaining of "insights"--- as there is rarely any insights to be gleaned from that sort of information flotsam. I can gain insights from reviews of old brains and comparing them--- particularly side by side--- with later brains on similar subjects. But you can't actually do that in PB (as of version 5.x) if you put it all in one brain--- as you can no longer have multiple instances of PB open to the same brain. So just to see your major network groups in a big brain, you have to export it or copy it to a new brain and then you can finally do side-by-side comparisons (although--- if you spend the time in Expanded View, you can set them up in it--- with a lot of arranging and pinning--- an extreme lot of bother if they are not very interlinked clusters).
I'm not claiming my way is the best--- other than it works well for me. But the reason it does is that by keeping the information isolated means that it is easy to search, and easy to know where to look for the info. And frankly--- depending on the information DENSITY, it might not even be in my PB. I use other tools for certain kinds of information storage. So if the information would be better SEARCHABLE in another tool, then it is probably in that other tool. (for example, I use Zoot to maintain an "information collective" of all the space related web articles I've found interesting for years--- so if I need to refresh my mind on a the later trends in cosmology or how a particular commercial launcher has spun its releases for a particular period, I go into it, and use its superior search capabilities to find it. I don't have to PRE-CATAGORIZE or otherwise PRE-MARK an item in Zoot--- I can build smart searchers on the fly to do that for me.)
Information organizers/Information management really have 2 competing goals:
PB meets the first goal easily and well with Instant Search, Pins, and Recent Thoughts.
- To help put the information you often need right at your fingertips. This first goal means you want all that info floating around, and zooming down to the user the instant they need it. So you want easy controls for the user to "recall" that info. Bookmarks, pins, recent used lists--- that's all mechanisms to put the information you use often, or you find so important you want to easily jump to it, right at your fingertips.
- to be able to find any item of information stored in the archive, and make it easy for the user to find that information with a minimum of knowledge remembered about it.
PB isn't so good with 2. It is trivial to look through a brain with just 30 thoughts, and see if any of those thoughts are what you were looking for. But if you have 30,000 thoughts, you have just scaled out beyond what you can easily or trivially find. If that item isn't marked in some way to make it easy to come up with a small subset of items that I can review to find it, it is good and lost. At this point, my information system has failed me when I really needed it. I don't need it for "quick launching" apps or web pages--- as there is literally thousands of apps that will do that for me. What I really need is a long term information retrieval system that is superior to my own mind. So to meet number 2, you need to mark items in as many ways as possible.
So how is 1 and 2 competing goals? To meet #2, you need to really mark up any new items--- and even revise older items with the new classifactions that you evolve as you add in new items and new markings you can (hopefully) easily find the information years in the future. This means the more items you have, the more markings you have (and you may have several different systems at work at once). But "the information you often need right at your fingertips" emphasises how often you want to access that bit of information, so you end up with just 1 or 2 ways in--- ie, thoughts linked to pins (or a thought that has been set as home). And that encourages you NOT to heavily mark items--- as you are just using a few hubs (or trunks) to find all the "important" stuff. This leads to piling up the "non-important" stuff so you can get on with doing what you need to get done (work, journaling, shopping, whatever).
In a purpose focused brain, the information in it has already gained context--- by just being in that brain. Furthermore, it will be organized in some organic fashion--- whether you set out with one particular kind of organization at the start or just had one form over the creation and use of that purpose focused brain. Also, it will have less items in it then an "everything" brain, so you will have less items to look through on your searches. And that is one of the main value of these tools.
It's something to think about--- in the future, once you've left that brain (or that area of the brain, if its a big brain), how will you FIND that information again in 9 years, when you've changed how you think and how you view the world? After you've had a while to forgotten frankly everything--- even the "significant" associations between thoughts in that brain? Or do you not care, because it (probably) won't be important to you then? Is it more important doing things quickly? Or do you need a balance between quick and being able to find things? Think about it--- as this will determine what will be the best way to arrange your brain.
Have fun, and enjoy your thoughts!