House of Lords - Reynolds v. Times Newspapers Limited and Others:
"The context is newspaper discussion of a matter of political importance. Stated in its simplest form, the newspaper's contention is that a libellous statement of fact made in the course of political discussion is free from liability if published in good faith. Liability arises only if the writer knew the statement was not true or if he made the statement recklessly, not caring whether it was true or false, or if he was actuated by personal spite or some other improper motive...
The common law has long recognised the 'chilling' effect of this rigorous, reputation protective principle. There must be exceptions. At times people must be able to speak and write freely, uninhibited by the prospect of being sued for damages should they be mistaken or misinformed. In the wider public interest, protection of reputation must then give way to a higher priority.
Honest comment on a matter of public interest
One established exception is the defence of comment on a matter of public interest. This defence is available to everyone, and is of particular importance to the media. The freedom of expression protected by this defence has long been regarded by the common law as a basic right, long before the emergence of human rights conventions. In 1863 Crompton J. observed in Campbell v. Spottiswoode (1863) 3 B. & S. 769, 779, that 'it is the right of all the Queen's subjects to discuss public matters'. The defence is wide in its scope. Public interest has never been defined, but in London Artists Ltd. v. Littler  2 Q.B. 375, 391, Lord Denning M.R. rightly said that it is not to be confined within narrow limits. He continued:
'Whenever a matter is such as to affect people at large, so that they may be legitimately interested in, or concerned at, what is going on; or what may happen to them or others; then it is a matter of public interest on which everyone is entitled to make fair comment.'
Traditionally one of the ingredients of this defence is that the comment must be fair, fairness being judged by the objective standard of whether any fair-minded person could honestly express the opinion in question. Judges have emphasised the latitude to be applied in interpreting this standard. So much so, that the time has come to recognise that in this context the epithet 'fair' is now meaningless and misleading. Comment must be relevant to the facts to which it is addressed. It cannot be used as a cloak for mere invective. But the basis of our public life is that the crank, the enthusiast, may say what he honestly thinks as much as the reasonable person who sits on a jury. The true test is whether the opinion, however exaggerated, obstinate or prejudiced, was honestly held by the person expressing it...
It is important to keep in mind that this defence is concerned with the protection of comment, not imputations of fact...
Lord Diplock. at p. 150, emphasised that indifference to truth is not to be equated with carelessness, impulsiveness or irrationality in arriving at a positive belief that it is true:
'In ordinary life it is rare indeed for people to form their beliefs by a process of logical deduction from facts ascertained by a rigorous search for all available evidence and a judicious assessment of its probative value. In greater or in less degree according to their temperaments, their training, their intelligence, they are swayed by prejudice, rely on intuition instead of reasoning, leap to conclusions on inadequate evidence and fail to recognise the cogency of material which might cast doubt on the validity of the conclusions they reach. 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.'"