Autopsy Table allows you to dig into corpses and find out what's inside. Yuck. Depending on the "inputs", you can get some Madness out of it.
Corpses (cadavers) are often dropped by NPC bandits. You can find them in some dungeons (such as Floran prison) or by accepting tenant quests like "deal with bandits". Performing an autopsy on 25 cadavers is enough to get 200 Madness that are necessary for the first node of Metaphysics tree.
There is a usable Autopsy Table at the Science Outpost. It belongs to Kevin, who totally doesn't mind when other people perform autopsies in his room.
Tips
If you have a lot of Madness stored inside the Autopsy Table, you can place a Storage Bridge near it, and then use Item Network Terminal to retrieve only the quantity that you need (instead of taking all of it).