What is the test for defaming a defamer to be found guilty of defamation?

The following passage from Horrocks v. Lowe (at p. 669) applies to this case: … What is required on the part of the defamer to entitle him to the protection of the privilege is positive belief in the truth of what he published or, as it is generally though tautologously termed, an “honest belief”. … But indifference to the truth of what he publishes is not to be equated with carelessness, impulsiveness or irrationality in arriving at a positive belief that it is true … but despite the imperfection of the mental process by which the belief is arrived at, it may still be “honest”, that is a positive belief that the conclusions they have reached are true. The law demands no more.