 Al-Fārābī goes on to define scientific knowledge as being accurate, certain, and steadfast in any given moment, which means it does not change through time and does not go in and out of existence. Certainty cannot contain doubt and falsehood, since what can in any way be false cannot be considered certain or be said to be knowledge. For this very reason, al-Fārābī points out that the ancients did not consider sensation of what can change as knowledge; rather, scientific knowledge is confined to the “certainty” about the existence of things that cannot possibly change. In order to explain this al-Fārābī offers the example of the number three being always an odd number, and how it cannot be otherwise.