The principal use of the simple present is to refer to an action or event that takes place habitually, as in He writes for a living (in contrast to the present continuous, which refers to something taking place at the present moment: He is writing a letter now). However certain verbs expressing a state, such as be and know, are used in the simple present even when referring to a temporary present state. There are also certain other uses (including those mentioned in the following paragraph) in which the simple present does not reflect a habitual aspect.