Brain software is kind of a fun idea of software that runs on the human brain as opposed to a computer. This removes the dependency on computers and highly increases freedom. Of course, this also comes with a huge drop of computational power :) However, aside from being a fun idea to explore, this kind of software and "architectures" may become interesting from the perspective of freedom and primitivism (especially when the technological collapse seems like a real danger).
Primitive tools helping the brain compute, such as pen and paper or printed out mathematical tables, may be allowed.
Example of brain software can be the game of chess. Chess masters can easily play the game without a physical chess board, only in their head, and they can play games with each other by just saying the moves out loud. They may even just play games with themselves, which makes chess a deep, entertaining game that can be 100% contained in one's brain. Such game can never be taken away from the person, it can't be altered by corporations, it can't become unplayable on new hardware etc., making it free to the greatest extent.
One may think of a pen and paper computer with its own simple instruction set that allows general purpose programming. This instruction set may be designed to be well interpretable by human and it may be accompanied by tables printed out on paper for quick lookup of operation results -- e.g. a 4 bit computer might provide a 16x16 table with precomputed multiplication results which would help the person execute the multiplication instruction within mere seconds.
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