I affirm it to be a cat and deny it is a dog. Merely looking at the pet and forming an idea of it is not knowledge that can be true or false. Truth and error are always found in the judgment. This is called mental or logical truth. It consists of the conformity of the mind to the thing.
Logical error is defined as a disconformity of the mind to the thing.
There is also truth in the things. 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.