Memory loss, forgetting something when you need it, or feeling that you are relearning something. You probably experience these frustrating sensations regularly in your life. In this article, I discuss the concept of building a second brain to help you cope with the challenges of long-term memory. Personal knowledge management (PKM) is a topic I am passionate about, so I hope that you will bear with me for the following general and software-agnostic introduction.

Behind the idea of a second brain, my argument will be specifically focusing on storing information on the long term for personal use (not restricted to your personal life though). Other uses, including organizational and collaborative knowledge management, hold quite different challenges which I am not going to cover in this article.

My adventure with personal knowledge management officially begins in 2019, when I finally acknowledged that my organizational method was hopeless and got started searching for a solution. I had been inspired by my father, who has been using a tool appropriately called The Brain ever since the middle of the 2000’s. At home, I was repeatedly amazed by the effectiveness by which he could retrieve so many kinds of information from his computer-based knowledge base. However, at that time I had specific requirements that made me exclude The Brain, notably its pricing and the absence of a GNU/Linux compatible version. I even attempted to build my own app for that purpose with some classmates back in my university days, which I abandoned. Ultimately, I started building my second brain in 2021 with TiddlyWiki, then a year later I switched to Obsidian.

The motivation

Let’s acknowledge that the human brain is impressive and unique on earth, doted with yet unequalled intelligence, creativity and memory. However, it has limited capacity, it is prone to mistakes and oversights. Behind building a second brain lies the idea of storing and structuring information that you want to remember, and in particular to reuse, on an external medium. It complements your primary brain by offering benefits such as quantity, clarity and longevity, thereby partially overcoming its limitations.

Having a second brain can help you be more efficient at manipulating knowledge, to better understand complex and large concepts. In consequence, having a reliable support for storing information boosts learning. Indeed, it becomes worth it to spend time exploring because you become confident that you will be able to retrieve the acquired knowledge fairly easily when needed. In addition, it can help to produce creative work like writing by building on top of solid grounds.

From my experience, my second brain system has really pushed me to grasp new topics, including bigger and complex ones related to science and technology in particular. Notably, it made me feel ready about going for a PhD and doing research. Working with my second brain was essential in practice thus far.

Traditional systems

Not everyone is familiar with the probably pretentious term of “second brain”, but that does not mean that people did not attempt to build one either. To dive in those past undertakings in history, I recommend reading this article about a theoretical machine called the “memex” by Vannevar Bush, which was first published in 1945. There is also a Wikipedia page about the Zettelkasten method which was invented in the eighteenth century. The commonplace book and personal knowledge database Wikipedia articles are very insightful to read as well. Despite historical evidence, those advanced organizations systems are not what the people would commonly implement.

Paper notebooks

The classic form of storage, and still very much often practiced, is the good old paper notebooks. At least up to my generation, we have been trained to use these from primary school and were never questioned.

While they have offered a decent medium for centuries, they are very constraining. Although they have a relatively big physical size compared to digital devices, one cannot store that much information in them. They start accumulating, whether their purpose is for saving lecture notes, writing minutes of meetings, or for creative thinking. Just think of all the different notebooks that you have used during your education for each schooling year and subjects, sometimes having several for each. Consequently, searching for something in notebooks is not straightforward, to find the notes you are looking for you must remember the related subject, year, page, and storage location. For practical reasons, knowledge workers cannot carry all of their notebooks everywhere they go. So they have to decide on the most relevant to bring, with the risk of being wrong and missing some information.

Another important consideration is that, in theory, most of the additional knowledge builds on top of previous grounds. Also, past erroneous or incomplete information should merely be updated than rewritten from scratch. Yet, with this physical medium, editing preexisting notes turns out overcomplicated and messy compared to starting all over again on a blank page. I should stress that this extra effort has at least one benefit, it can help to memorize. Unfortunately, we do not have the leisure to go through this process every single time we have to update information.

Finally, structuring knowledge on a per subject basis has limitations. Many concepts cross-cut disciplines and possible connections between them are harder to represent and exploit using physical notebooks.

Digital notebooks

Gone are the days when paper was ruling the world of information. Digital devices have replaced a large part of that in our personal and professional lives. Then, how do we make use of this technology with regard to knowledge management? People started to use Word documents (or alternatives), Evernote or even simple text files. We have also seen more advanced tools popping up in work environments, like OneNote, Notion or other personal wikis. But they tend to be used as mere virtual replacement of notebooks, haunted with many similar limitations. Even modern applications like Notion and OneNote maintain a folder structure that replicates the archaic physical storage (is this the digital revolution?). We are offered to store notes in folders and subfolders (e.g., OneNote pages are stored in a specific section, itself located in a specific notebook). In general, even with a digital-based knowledge system, things can get lost quite easily. Information can be scattered in many locations on the device, on multiple local, external or cloud drives, not to forget several accounts and digital devices.

The single-path access logic caused by the tree-like organization system tends to add some overhead when saving new items. You have to think of where the information should be best located, or rely on duplicates to have it in many places, with the problems that come with such solution.

I advocate to get away from those limitations and opt for a system based on connections. In fact, you should not be forced to remember where each note is located, nor its precise name.

Some key concepts

Capturing knowledge and structuring it is part of the thinking and learning process. Behind this concept is the idea of maintaining evergreen notes that are continuously reviewed and updated. Yet, we are more familiar with temporary notes, like those taken during a meeting, which are not meant to be updated and from which it is harder to build strong knowledge. We should see our notes as art: they start as a vague idea, turn into a draft and get gradually polished. I like how some people refer to this as “digital gardening”. 

The power of the digital format is that information can be virtually anywhere because links are still easy to make and allow travelling instantaneously to the other location. In addition to direct links, bidirectional linking becomes possible, where the connection is visible both in the referring note and the mentioned note. The latter note is “aware” of all the other notes referring back to it (these links are called backlinks). Ideally, some context is attached next to each backlink (e.g., note origin and text content). They make it possible to create some virtual structure by “parenting” notes by linking to those other notes to which they are related. Consequently, the mentioned notes become one of the possibly many parent folders which can later be used as entry points to the mentioning notes. This ultimately makes it possible to access a single note from different paths, depending on the mental associations used, just like if it was stored in those many places while still being the same note.

In addition, linking can help to reduce the amount of information stored in a single note. It permits to avoid duplicating information across notes but linking to the shared topic instead. Just like on Wikipedia or the web in general, hyperlinks make you aware of the additional available information, you can choose to explore it or not.

In contrast to some big voices among the second brain community, I do not try to maintain “atomic” notes being very specific and small. I want my notes to have the content that I find the most meaningful and avoid scattering things more than necessary. Whenever the note length gets impractical, both in terms of readability and reuse potential in other notes through links, I start considering breaking it down into smaller notes. In consequence, when the knowledge on a particular topic grows in my system, it becomes ideal to implement indexes, also called map of content. Those work by maintaining entry-point notes that structure large topics, those indexes give an overview of the different notes that are related. They can be structured and adjusted on the go, and also be automated by leveraging the backlink capabilities (i.e., listing notes mentioning a specific parenting note), some tools offer advanced querying systems to fine tune this listing of links. In my use case, I do not strictly separate index and notes. Instead, I tend to insert references to any other relevant notes I may be looking for when later reading the note, whether they are related topics or subtopics. Links are critical for retrieving older notes when you do not remember the name, location or its existence, which is one of the main reasons to maintain a second brain.

While navigating notes via links is one way to access the information, the classic search function must not be neglected. In fact, this is still the approach I use the most to access my notes. When it is possible, searching for notes via their name is like directly calling them. Nonetheless, giving a good name, that is easy to remember, short and clear is not straightforward, especially in the case of new and complex topics. This is the reason why having a way to give your notes as many names, or aliases, as you like is a must-have for any personal management tool. For example, there can be several synonyms for a concept, and you would rather not try searchig them one by one until there is a match with your notes. Not to forget about multilingual use and the different translations! Though, I do not give my notes all possible names, just the ones I use or frequently used in my literature.

Having said all this, sometimes I struggle to find information I know is stored in my vault. When I finally find it, I learn from the experience by updating the note, giving it a better title, additional alias and reinforcing its links for the next time I need it.

I am not fully against some structure, I even advise having at least three different folders. One where all stable information is stored, which eventually contains most of the notes. Another one that serves as an inbox, where all new notes automatically end up by default. This separates the process of recording and organizing, thereby relieving some pressure to focus on the note itself. This also helps when you are not certain whether you will keep the note, by separating it from the vast realm of already existing notes. Finally, I recommend having a temporary folder, where you can explore some features and put short-lived notes (e.g., editing a duplicated note before export without altering the original version), basically serving as a sandbox. You could virtually delete the content of this folder at any time without dramatic consequences. If your system supports such functionality natively, an additional fourth folder serving as an archive could be ideal. This is similar to the recycle bin we have on our computer, which can minimize the risk of unrecoverable mistakes induced by direct permanent deletion.

What should be stored

You want to store any information that is likely to be useful in the future, especially when it is more efficient to have it in your second brain than to spend time searching from external sources once again. This has to consider the time it requires to record, structure and later search for it from your system versus how long it takes for both searching and comprehending third-party information. Nevertheless, some information may be impossible to access by any other means than having it stored. This includes personal communications from people who are not reachable, explanations coming from books you do not have access to, or which are simply too massive to quickly skim through. Having saved the information is also critical when you are away from an internet connection or when you need very fast access to it. My second brain often helps me in reexplaining difficult notions in a way that is much clearer and faster way than if I had to get through an external source again, the information is already processed. Knowledge acquired from your own understanding, based on inferences, is also particularly suitable. It is not because I have understood or inferred something in the past that it is going to last forever, in such cases I have better securing it in my second brain. My personal reflections are another kind of knowledge, which could be simple additional annotations that add extra meaning to some reference.

In general, when dealing with any interesting new information, I ask myself: “would the future me prefer finding this in my second brain, or searching for it again from external sources will be easier, faster and more robust?”. Put differently: “Is it worth it to save this my second brain?”

I would rather not waste time, effort and memory, so I naturally end up selecting, similarly to how the brain filters information. We cannot simply store anything we find, this would take an infinite amount of time and storage resource. I think that whenever some information could potentially be helpful in several years, it makes sense to save it. You should also focus on the minimum yet meaningful fragments worth being saved out of a bigger reference. Time does not allow us to reread entire books or any other references to get some information again. Distillation of this knowledge is the following step, you want to extract the smallest fraction of information that is relevant for you and to disregard the rest. I generally do this refining iteratively:

1. I collect a reference (e.g., borrowing a book);
2. While I process it I would annotate passages that seem interesting (e.g., highlighting a PDF, taking notes during a presentation);
3. Then I extract what is worth keeping on the basis of what I already have and what I need and save it on my second brain;
4. When rereading those notes later in my knowledge base I would consider removing irrelevant details or in complementary ways to highlights key ideas within for future fast skim through;
5. I would sometimes go one step further and add some short bullet points on top of the note to summarize the key ideas. This both helps me to put things into perspective and for future quick use of the knowledge.

This process is called “progressive summarization”, an idea which is presented in Tiago Forte’s book “Building a second brain”.

With experience you will tend to get more sense of what is relevant. Moreover, this selection has to remain agile. I would sometimes not be able to properly judge about the future potential, so I just relax and allow myself to exceptionally save it anyway, it will remain possible to remove it during a future cleaning. Using an inbox folder, as previously mentioned, limits the chances of flooding your knowledge base if those situations are frequent.

I also personally prefer to distinguish store short-lived information and long-term memory. The former can accumulate very fast, and I prefer not having many notes laying inside my second brain with quickly outdated information. Short-term includes live notes (e.g., meeting minutes), to-do lists, project management, etc. I am maybe overcautious on that, so do not take it rigorously!

A record, if it is to be useful to science, must be continuously extended, it must be stored, and above all it must be consulted – Vannevar Bush

A second brain complements your biological memory

You should not passively feed your second brain with content, but be thoughtful on building meaningful notes. Involving your own brain in the process indirectly trains it, just as we advise students to take notes to keep them active in the learning process. Copy-pasting what you read or ear is still quite passive. You should pre-digest information as much as possible. Make sure to understand it, highlight the core of the key information, while removing repetitive or uninteresting stuff.

Knowing about what you know is also crucial. Indeed, I frequently find myself facing challenges that I already faced in the past. In such case, I quickly search my second brain and frequently surprise myself with preexisting information ready to help me. I want to emphasize that disposing of some meta knowledge is probably more important than remembering the actual content of the notes themselves when it comes to knowledge management. When the information is stored clearly, one just has to retrieve it and exploit it when needed.

Train your mind for natural recall in parallel, which is still necessary in situations where fast answers are needed (e.g., casual conversations, meetings, or when live understanding or explanations are expected). To improve it, there are some complementary methods, and I particularly like the spaced repetition learning technique using flashcards. Your second brain would still contain more information in quantity. I recommend reading this nice comic strip to learn more about spaced repetition and how it can help you remember things. I learned more than 450 words, concepts and definitions in less than a year by spending less than 5 minutes per day using Anki!

All in all, keep the right balance between biological memory and your second brain.

One cannot hope thus to equal the speed and flexibility with which the mind follows an associative trail, but it should be possible to beat the mind decisively in regard to the permanence and clarity of the items resurrected from storage. – Vannevar Bush

Practical considerations and cautions

As said earlier, saving information and structuring it in a second brain has to serve a purpose and be used ultimately. Otherwise, you are probably going to lose your time. The more you use a second brain, the most efficient it becomes. You train at finding what you need while you improve it when it fails to do it for next time. In general, maintaining a knowledge base takes time. Just like you take care of your physical belongings, you should also do it for your digital garden. This means frequently reviewing parts of the second brain, split or merge notes, clean outdated content, add missing links, and get an overview of your available knowledge. This process helps in getting some hindsight about your meta knowledge, which I previously mentioned. It pays off without you realizing whenever you use it, but again keep it reasonable as I advise next.

An important warning if you want to stay productive is to be careful not overengineering your system passed the point of diminishing returns. I often see people expressing regrets regarding this issue in the productivity community. There is so much theory, workflow examples and powerful tools available that it can be a never-ending process to build your dream system. While I believe that a minimum exploration and training is indispensable, it should remain limited to start exploiting the system fast. You have to try, fail and discover what works best for you with experience. When you are satisfied with the minimum requirements, stick to it as much as possible. It will definitely feel incomplete on the edges, but perfection is the enemy of good. Having your most recent notes together will already be beneficial fast. For most of us, knowledge management remains a means to get the work done, not an end in itself.

I have mentioned in the introduction that this article will not address collaboration. In my opinion, sharing your second brain as a whole with others is impractical and should not be aimed for in the case of personal knowledge management. There is not yet a “one size fits all” software or method, both personal and collective knowledge management have different use cases. Librarian and archivists have developed systematic classifications to allow their colleagues and users to retrieve information by agreeing on some classification rules. But such a system has to be learned and followed strictly. It can be very frustrating when a certain item has been classified under a different category than the one you would have naturally thought of. What we are looking for when building a second brain is to free ourselves from conventions and take benefit from our intimate association of thoughts. Yet, I manage to share notes occasionally by doing exports of single notes. But even this could be altered by the way you organize the content of the notes. What may speak to you may be unclear to someone else. You would rather not overload your notes with additional contextual information not useful to you that will dilute the key messages. My advice is to first take the time to learn on your own, improve and when you are ready to go out contributing in the outside world, all on top of the solid grounds via your physical and second brain. Again, this is an opinion, not a rule!

One of my biggest, yet easy to achieve, advice when it comes to storing knowledge is to always give it some context and source. I am doing a PhD so it may sound specific to academics, though I realized the importance of doing it even before jumping in my research journey. In fact, I seldom mentioned the source of facts and stored them raw. With time, I started to look back at my older notes, I found myself uncomfortable with content for which I could not judge the reliability or the origin. I would not even be able to differentiate my words versus something copy-pasted from an external reference. Consequently, I strongly recommend yo to always mention the reference to (almost) any content you put in your notes. Along with the title, I save the year of publication for journal article and books or the access date for anything found on the web with the URL. This also holds for personal communications (e.g., a discussion with a colleague, an email). When it is a personal thought, I still date it and mention myself to make it clear for the future me reading it. This helps to judge if the information can still be considered valid, if it should be taken with caution or blatantly outdated.

A final consideration to bear in mind is about control of the data. My vision of a second brain encourages putting all pieces of my long-term knowledge at the same place. Yet, centralizing information increases the risk of losing it all if not properly managed. That is why I recommend implementing a backup process in parallel, look at implementing the 3-2-1 principle like IT engineers. Avoid tools that bind you to their system, by not giving control to your data and using proprietary formats. We should keep in mind that no software is made to last (Google proved it often, same for Microsoft), so at least make sure the underlying data can be migrated without overwhelming difficulty. Be careful with software with a lot of functionality that cannot be reproduced in alternatives or from which some information cannot be extracted.

A few personal and professional examples

To clarify the abstract concepts introduced in this article, I wanted to provide some illustrative examples, so you get to see situations where my second brain is useful in my daily life. I separated my personal and professional use cases, even though sometimes they can be strongly related.

Personal use cases

• Gift idea list. I feel terrible at having creative and good ideas for gifts, plus I am against consumerism. I want to offer useful things that people will have use of. To support this I eventually started to plan surprises by recording ideas whenever they appeared, not waiting to the last minute with nothing great coming up.
• Writing this article. I am passionate about the topic of knowledge management and I have read numerous books, articles and watched videos about it. Many key ideas were recorded in my second brain that I eventually compiled in this article. For example, the quotes from Bush were taken from it.
• I write about the history of my relatives. There is a lot of emotional value in what my loved ones tell me from their life experiences and stories. When they will no longer be part of this world, I would still be able to remember and get inspired from them. The idea of forgetting what they once told scares me. Similarly, I write down inspiring pieces of advice so that I can recall them in the future and reflect on them with my experiences.
• Book annotations. When I read an inspiring book, I would save short exerts that summarize the key ideas I want to remember. Otherwise, it would get blurry fast and may eventually be forgotten. I would rather not reread them to explore others for the sake of crossing point-of-views and growing my knowledge.
• I document what I learn from tutorials and experience regarding bike repairs. There are issues that are recurring, but not so often, though I often forget in between two occurrences. Meanwhile, many tutorials are incomplete regarding the instructions they provide, so my notes mix the procedures to have a more complete and consolidated procedure that fit with the specifics of my bikes.
• Cooking recipes. Similarly to mechanics, recipes are interesting, they are usually adapted to my taste and habits. When following a recipe I discover alternatives by trial and error and document them, offering new reproducible steps or different ingredients. Also, many originate from oral communication. One example is for the couscous recipe of my grandma that she only knows on top of her head. She once gave a first version, and when asked again months later for verification, she added other details and forgot about others she previously told. Eventually, I was able to get the jigsaw puzzle complete. Another example to illustrate the use of links, recipes are sometimes split, with some parts used in several preparations. For instance, the sourdough bread recipe links to the sourdough starter recipe, which is also the case in the brioche recipe (sweet bread), whereas the pizza recipe and olive bread (fougasse) link to the sourdough bread recipe as being mere adaptations.
• To give a final personal use case of my second brain, I try to be better aware of myself. My digital garden is a place where I reflect on my goals. I once also thought about my qualities and flaws, I listed them down with some example situations to help me think about them. I later realized that it was useful to track change when I reread them several months later.

Work related use cases

• At the crossroad of personal and academic knowledge lies my reading notes. When I read something interesting, whether it is a book, a scholarly article or any document, I save the interesting extracts in my second brain. If I was not doing this it would probably get forgotten quickly. Even though these files are digitally annotated, remembering the right references is not guaranteed among the hundreds of documents I check every year.
• My second brain serves a support for learning new scientific methods. For example, I have used it to learn input-output analysis and life cycle assessment, two large and important sets of methods related to my PhD research. Matured notes now help me a lot when I deal with one or the other to recall some aspects of the method.
• I use my second brain to learn Python libraries extensively, one example is with the “pandas” library. I build my own complementary documentation. There are many cases where I found some solutions via web search, provided by StackOverflow answers or some random technical blog, not to forget AI-generated code snippets. I saved such code snippets that worked and seemed to be possibly useful again in the future, while carefully documenting any additional information to make it clear for me, along with the source. Similarly, I used it to learn VBA several years ago, recently I wanted to use a script found on the internet, which led me to reuse older notes to help me understand and fix the script. Another frequent and similar use is to document complex Excel formulas that I do not want to learn by heart.
• Through my bibliographic work, I had to re-explore a topic about biomass pretreatment. I already read some articles and carefully saved key understanding more than a year before about this. I reused my notes and it was much faster and accessible to grasp than going through the publications again. Furthermore, I eventually got a new level of understanding by looking at my notes and trying to refactor the structure to make it more meaningful, realizing some discrepancies in the way some authors classified things in their article.
• I am writing my first research paper, and the content I saved in my second brain helps me a lot to leverage the references I read all along my initial PhD year, which were organized in my second brain. Having bibliographic context next to facts really helps.

Uncertainties and future perspectives

Even after several years of experience, I still have some uncertainties regarding my knowledge management workflow. For the sake of transparency I am sharing those along. This will highlight once again that building and maintaining a second brain is an ongoing process.

I still have not got a proper archiving system to handle outdated, unused or content that is simply not useful after all. I tend to accumulate more than I possibly need. Occasionally, I would remove content, but only when I come across them. I should probably take the habit of doing recurring reviews (e.g., once a year, when having some spare time). Similarly, I try to clean pictures on my phones or clearing my mailboxes regularly. I would have better doing it on the go as much as possible to reduce those clean-up session efforts.

Another consideration is the use of artificial intelligence (e.g., LLMs) related to processing the knowledge in my second brain. This kind of applications started popping up quite fast in the past few years. The idea is the AI would be more efficient at summarizing your accumulated knowledge than you looking up for it, just like AI is being used to replace search engines. It can potentially help by discovering unthought connections in the knowledge base. However, I did not implement AI for two reasons: The first is that some content in my second brain is private, I am not OK to blindly share my entire knowledge to big tech companies; The other reason concerns the sustainability of AI, I would rather not generate yet another use of AI.

While the dream is to have all of what I need in the same place, efficient and searchable, I still separate concerns. I see my second brain as a place for long-term knowledge and I complement it other tools for things related to project management, personal to-do lists, official documents, photos, work files and manage references (e.g., books and articles). You can read about my tools in this article. It could probably be improved and the file management done more systematically, but this works fine for now.

Finally, I would like to get more insights regarding my use of the second brain. Having some statistics regarding the content I use the most and the least could help me see where is my most and least relevant use of my second brain. It is something I may explore in the future.

Despite those limitations I do not try hard to come up with a fix. If they become important in the future, I might consider searching those solutions. I would rather not waste time looking for a solution that is just waiting for a problem.

A final encouragement

As we reach the end of this article, I hope that you have learned something and that I sparked your interest in building your second brain.

If you work in academia, still a student or just enjoy learning, I think that building a second brain will suit you well. This is the type of tool you probably need to cope with large quantity of information to grasp.

Keep in mind that your system will always be imperfect and evolving. You should not wait until you find the ultimate tool and method that fits perfectly with your ideal system. Just get started with one that checks your minimum requirements. The rewards come with time, throughout the experience you will gain and based on the accumulation of interconnected relevant knowledge. I find it so satisfying to read notes from several years ago, which constantly remind me of the power of maintaining a second brain.

Keep in mind that a second brain is not just about having a knowledge management tool, it is primarily a mindset: You want to organize information in such a way that you make it easy to retrieve and reuse in the future, the tool has to serve that purpose. In essence, see your second brain as a gift to the future you.

Now, if you are eager to know more about my experience with using Obsidian to build my second brain, I invite you to proceed with the next article. Feel free to share any feedback about this article, in the comments or to discuss with me about this directly!

Appendix: Short overview of apps available

Getting towards practice after theory, I want to share some apps that can be interesting to explore. This is not an extensive overview but my two cents about different software, what they have to offer and their limitations (in my opinion).

• Obsidian.
  • This is my second brain app where I store and organize my medium- and long-term knowledge. I trust it as it works offline by design, based on markdown files which offer an excellent level of interoperability. I love its deep customization capabilities and the backlink functionality built into it. It is compatible with most operating systems (I used it on Android, Ubuntu, Windows 10/11, MacOS), with a well-supported mobile version. Note that it does not offer file synchronization out of the box. You can read more about it in the article I wrote specifically about my experience with Obsidian over here.
• Logseq
  • I did not use it, but I know a bit about it. It is a good and powerful alternative to Obsidian. It is amusing that they were both initially released in 2020 and are very similar. Although it has much fewer plugins to offer at the time of writing, it has a growing community, which I find important for getting help, inspiration and new features via community plugins. Like Obsidian, it is offline-first, with a highly customizable interface. In contrast, Logseq is open-source software. It also has a mobile application, but it is more limited than the desktop version.
• Roam
  • I never used it, tough it is famous in the realm of modern PKM. It has probably influenced Obsidian and Logseq. A big difference is that it is a web-based application (Software-as-a-Service, SaaS) and I do not trust this model on the long term because the company has to stay alive for the service to work. This is a subscription-based system with no free plan, and I find it not cheap. I am uncertain about its long-term perspectives and strength to keep up with the competition.
• Notion
  • I use it as my short-term knowledge base for my PhD and have several years of experience with it to organize my work-related projects, temporary notes and tasks. It is not so new any more, but it managed to keep up with the competition. They really brought a positive change into the world of productivity apps with a good design (UI) and user experience (UX). It is a web-based application, with free and paid plans (note that I get the paid plan for free as a PhD student). It has remained quite healthy economically speaking, but I would still not trust a SaaS for a long-term second brain. Notion has only limited support for backlinks, although I have the feeling that it will improve in the future.
• OneNote
  • A big player, extensively used in the tech industry and in academia. However, to me it is more of a digital replacement of paper notebooks than a real knowledge or productivity app. It is ideal for short-lived knowledge and has neat features for creativity.
• Personal wikis: MediaWiki, XWiki, TiddlyWiki and many more.
  • You can set up your personal wikis as a personal knowledge management system. There are commercial options such as Atlassian Confluence, which I tried before in the context of a big IT company. I really liked the powerful full-text search.
• The Brain
  • This one is a special kind, which inspired me in building a second brain. My father had been using it for almost two decades as of the time of writing. This is an impressively old, but still powerful and quite niche software. Although it is a serious option for building a second brain, offering offline mode by design, its expensive subscription model did not fit with my needs.

There are many other apps around that I am not even going to mention, with many that I am probably not event aware of.

Note on Software as a service (SaaS)

Today, many apps work on a Software-as-a-service (SaaS) model, meaning that even a part or the entire interface is delivered via internet instead of running on your local computer or smartphone. When I started considering building my second brain I did not have mobile internet because I was living in a place where it was costly and limited. Despite that I now have a decent internet connection, it is not always available. The worst concern arises when the SaaS software provider itself fails. Outages happens, small to big tech companies are subject to it, and it can go as far as loss of data. I want to avoid the pain of not having my data when I need it. It sounds weird to me that it requires an internet connection to access my knowledge, like if my brain was not located in my body.