ITT: Post old textbooks that are worth reading and discuss.

Post anything from the obscure to the influential or from the scientific to the laughable (nowadays).
Whether it's a tractate, treatise, monograph, dialogue, essay, compendium, thesis, dissertation, etc., doesn't matter — post it here.
>What do you classify as old?
From Antiquity all the way up to about the early 1900s but anons can mess around with this definition if they want.
>Why old?
No particular reason. Reading these textbooks is mostly an exercise in /his/torical curiosity and aesthetic appreciation.
Although, science does with time, shift in which philosophies are predominant so it's worth having a wider philosophical scope than just believing in the prevailing theories of our time and accepting whatever your proff. taught you.
It's also not unheard of that some earlier scientific work greatly influences some later development.
Also some works are just classics that everyone should read.
>Aren't they outdated?
For math no. For physics probably but depending on when it was published, not grossly so. For pretty much everything else most likely.
I'm sure anons will mention if they are but outdated theories might be interesting in their own right.
If you're new to a topic and want an introductory work to it, it's usually a good idea to stick to newer works.

I'll start with the obligatory. Read it when I was 8 (a different version though, obviously. Got it from a library, it was published in the 19th c.).
An absolute classic, do recommend.