We have, for example, a very definite idea of the pet called a cat. The idea of cat involves a number of subordinate ideas regarding body structure, appearance, and so forth. This idea is a norm or standard to which a pet must conform in order to be designated as a cat. If the pet agrees with this standard, it is a cat, otherwise it is not. When objects conform to a recognized mental norm or standard, they are said to possess truth of being or ontological truth.
Ontological truth is defined as the conformity of a thing to the mind.
Reversely, ontological error is the disconformity of a thing to the mind. Now that all this is understood, we can define truth in general. It is the conformity between mind and thing. Conversely, error in general is the disconformity between mind and thing. Doubt Man is certain that he possesses knowledge about many things.