Obsidian is a cross-device tool for reading and editing Markdown files. Its central feature is functionality to make it easy to add links to other files in a note with simple double brackets (i.e. name of another note.
As a result, you can accumulate information and your own thinking together in a way that is heavily inter-linked, somewhat mimicking the way the brain makes connections between information. This allows for the slow development of ideas and arguments over time as the connections are developed and curated.
Importantly, you Own your data (the app sits on top of normal markdown files saved on your computer) and it is open architecture (can develop your own automatons and workflows on top of it). As the Obsidian CEO puts it, File over app (by Steph Ango):
File over app is a philosophy: if you want to create digital artifacts that last, they must be files you can control, in formats that are easy to retrieve and read. Use tools that give you this freedom.
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Obsidian is the base of my projects, writing, research, and episodic content (i.e. My life system).