Properly an attitude of the mind assenting unreservedly and fearlessly to a proposition, without thinking that it is possible for it to be mistaken.
This is the natural state of the mind in the presence of evident truth.
Under this aspect it is described as subjective certitude, or simply certitude as a quality of a proposition or of a thing, it is termed objective certitude, or certainty.
The difference is apparent in the statements: "I am certain," and "It is certain."
New Catholic Dictionary