Knowledge A Sim attempting to summon aliens with a telescope
Knowledge is an Aspiration in The Sims 2. Knowledge Sims are full of curiosity and always want to learn more and more. They always want to learn not only skills, but also other things that allow them to discover particular stuff, such as discovering secret vacation lots[TS2:BV].
Knowledge Sims' curiosity make them fear nothing. They always want to see Ghosts, become a creature, and see Aliens. They also like to encounter a near-death experience, such as defeating the Grim Reaper or being saved from death. Teenage Sims are eager to have good grades at school, and Young Adult Sims always want to be serious in their studies at college[TS2:U].
In the Windows and Mac OS X versions, the Sim will fear losing their creature status. Mac OS X has the same Wants and Fears as Windows with the exception of Wants and Fears introduced in FreeTime and Apartment Life since those 2 EPs were only released for Windows.
Knowledge Sims make good love matches for Family Sims, as they will usually want to get married and have at least one child.
Knowledge Sims are also the only Sims who do not fear becoming a special being (Vampire, Werewolf, etc). In fact, it can be used to boost their aspiration, curing it when it is no longer a fear to be cured. The want to become a special being is probably due to the fact that they want to know more about them, and what better way to learn about them than by being one?