Distinction between fact and opinion has been formulated by Bowen LJ in Smith v Land and House Property Corporation (1884) 28 Ch D 7, at page 15:
It is often fallaciously assumed that a statement of opinion cannot involve the statement of fact. In a case where the facts are equally well known to both parties, what one of them says to the other is frequently nothing but an expression of opinion. The statement of such opinion is in a sense a statement of fact, about the condition of the man’s own mind, but only of an irrelevant fact, for it is of no consequence what the opinion is. But if the facts are not equally well known to both sides, then a statement of opinion by one who knows the facts best involves very often a statement of a material fact, for he impliedly states that he knows facts which justify his opinion.