Knowledge about how each concept is linked to other concepts, and knowledge about how the features associated with a concept are linked to each other. People will reason differently about natural kinds and artrifacts - because they have different beliefs about why categories of either sort are as they are. Sometimes the symptoms caused by brain damage are specific, with some patients losing the ability to answer questions about fruits and vegetables, but still able to answer questions about other objects. Seperate brain systems are responsible for different types of conceptual knowledge. People will change how they use their concepts if their background knowledge changes.