The principle of sufficient reason states that anything that happens does so for a reason

Reason

Reason is a term that refers to the capacity human beings have to make sense of things, to establish and verify facts, and to change or justify practices, institutions, and beliefs. It is closely associated with such characteristically human activities as philosophy, science, language, ...

: no state of affairs can obtain, and no statement can be true unless there is sufficient reason why it should not be otherwise. The principle is usually attributed to Gottfried Leibniz

Gottfried Leibniz

Gottfried Wilhelm Leibniz was a German philosopher and mathematician. He wrote in different languages, primarily in Latin , French and German ....

, although the first person to use it was Anaximander of Miletus.
The principle has a variety of expressions, all of which are perhaps best summarized by the following:

• For every entity x, if x exists, then there is a sufficient explanation why x exists.
• For every event e, if e occurs, then there is a sufficient explanation why e occurs.
• For every proposition p, if p is true, then there is a sufficient explanation why p is true.

A sufficient explanation may be understood either in terms of reasons or causes for like many philosophers of the period, Leibniz did not carefully distinguish between the two.