Most knowledge requires complex forms of learning and several types of memory that serve different functions, localized in different parts of the brain. What we commonly call memory refers to the declarative or explicit memory for language. This memory is the core of propositional knowledge, mainly localized in the left frontal and temporal lobes of the brain; it is affected early on by Alzheimer's disease, which damages the hippocampus, a structure that plays an essential role in memory consolidation. In contrast, the memory for sensorimotor skills, such as cycling or swimming, is language-independent and involves the extrapyramidal system affected by Parkinson's disease. The physical encoding of different types of memory in distinct brain circuits contradicts the popular belief that knowledge is a non-physical psychological process.