Optimism is a mental attitude. A common idiom used to illustrate optimism versus pessimism is a glass with water at the halfway point, where the optimist is said to see the glass as half full and the pessimist sees the glass as half empty. The term is originally derived from the Latin optimum, meaning “best”. Being optimistic, in the typical sense of the word, is defined as expecting the best possible outcome from any given situation. This is usually referred to in psychology as dispositional optimism. It thus reflects a belief that future conditions will work out for the best. Theories of optimism include dispositional models, and models of explanatory style. Methods to measure optimism have been developed within both theoretical systems, such as various forms of the Life Orientation Test, for the original definition of optimism, or the Attributional Style Questionnaire designed to test optimism in terms of explanatory style. Variation in optimism and pessimism is somewhat heritable and reflects biological trait systems to some degree.[3] It is also influenced by environmental factors, including family environment, with some suggesting it can be learned. Optimism may also be linked to health.