Not everyone knows that. Some insist they can cast spells and are actually quite convincing but anything they can do you can do.
HD/Saves/Attack/Advance as Thief/Specialist.
You are well read and well practiced in your many talents and that's it.
When you encounter a problem - a monster, a locked pyramid, a riddling illusion, a crystalline barrier, an uncooperative sheriff - you can roll 1d100. If your result is equal to or less than your Intelligence plus your level then this is something you know about or know how to do. Your Int+Lvl is called your Theory.
You can use this for languages but a positive result doesn't imply fluency, it just means you know how to convey the specific message you want to say or decode this specific cave pictogram.
At any point you may add any number of Experience Points you've earned to your Theory as a one-time bonus before rolling d100 to determine your success. You level up more slowly but learn quickly.
If you ever want to deduce how to duplicate the effects of a spell or seemingly supernatural effect you've witnessed then you must roll against your Theory but may not add any XP to boost your chances. You get 1 chance for each spell/effect until you get another chance to observe it.
If you want to actually put your knowledge to use you have to make another roll to see if you can actually walk the walk or if you're just full of book smarts, not ready for the real world. This is a d100 roll against #x2, where # is the relevant Ability Score. #x2 is called your Practice. You may not add XP or your level to Practice but you get a +1 bonus to Practice cumulative the longer you try to grasp this new technique.
Once per level you can take some trick that you've successfully Practiced and turn it into a Technique. Techniques are a flat d20 roll equal or under Intelligence.
You have to pay to learn how to do stuff, you have to work to actually get good at doing stuff, and after that it becomes routine and memory.
The big control on this is that pulling off a lot of Techniques requires a lot of preparation or a lot of materials. A good Wizard has a whole lot of junk and tools on them at all times in order to better improvise but sometimes actually trying their Practice roll or executing a Technique will be just impossible without a trip to the store or some rare resource being involved.