That which is certain is fixed or stated, precise or exact, free from doubt or question.

Explanation

(English-origin maxim.) Legal certainty requires that rights, obligations, and commands be stated with precision sufficient to be understood and obeyed — a law or instrument so indefinite that reasonable men must guess at its meaning fails the standard of certainty and cannot bind the People.

Where this applies

• A contract requires delivery on 'a date to be determined later' — the court holds the term is too uncertain to enforce because nothing is fixed or precise.
• A will leaves 'some of my property' to a friend without specifying which property — the bequest fails for lack of certainty.
• A lease states the rent will be 'a fair amount' without naming a figure — the term is not certain because it is neither fixed, stated, nor free from doubt.