The first thing you will need are Fragments of Knowledge which are obtained by researching raw materials, crafted items, or Artifacts. Researching fragments will often produce Theories, which will in turn produce Discoveries. Right-clicking this item from the hot-bar will unlock the discovery, which will then be viewable in theThaumonomicon. ie. Discovery: Thaumic Restorer.
Before you activate this item, the recipe listed is un-craftable.
There are many theories and all but a few are acquired via researching fragments or by chance by skipping the fragment stage entirely(small chance, higher chance with artifacts). Some theories, however, require researching a very specific item to unlock, and also require that you receive the theory directly (no fragments). ie. Zombie Brain -> Theory: Brain-in-a-jar -> Discovery: Brain-in-a-Jar
Theories can have different levels of difficulty that are semi-random.
Success rate is with no bookcases or BIAJ s, you need to add items to boost this number.
Theories will always have a 100% loss chance, this applies after the theory is successfully researched and is desireable as theories are useless after obtaining the discovery (providing that you read the discovery immediately after receiving it of course), the loss chance in this instance is only applied to any additional items used to boost success rate.
Bookcases and Brain-in-a-jar slightly increase success rate and speed of researching.
Brain-in-a-jars also slightly decreases failure rate and loss chance.
You cannot discover the theory for an item if you have not already discovered the theory for an item you need to create it.
The Discoveries gained through studying lost knowledge are fairly everyday and mundane - as far as things can be mundane in Thaumcraft. You can expect to gain improved versions of existing tools, new uses for common items or other simple but useful items.
When you delve into forbidden knowledge you can expect to discover dark and slightly suspect knowledge. Things that are best not known, but are oh so tempting. Necromantic arts, dark sorceries and evil versions of existing items are all possibilities in this avenue of research.
Forbidden knowledge can be obtained by researching the remains of the undead, forbidden artifacts, or other items from the forbidden discovery tree.
Who were the builders of the monoliths? What ancient secrets do they hold? Well, don't just stand there - get to your Quasitum and find out! Of course, such knowledge might not have been meant for the mortal mind...
Eldritch knowledge can be obtained solely from eldritch artifacts and anything eldritch in nature (Void Ingots, Eldritch Stone, etc).