GLINSKI v. MCIVER [1962] A.C. 726

Lord Devling in Glinski v. McIver [1962] A.C. 726 at 766-767, as giving perhaps the most helpful definition or reasonable and probable cause:

…….. means that there must be cause (that is, sufficient grounds …) for thinking that the plaintiff was probably guilty of the crime imputed: Hicks v. Faulkner [(1878) 8 Q.B.D 167]. This does not mean that the prosecutor ha sto believe in the probability of conviction: Dawson V. Vasndau [(1863]. The prosecutor has not got to test the full strength of the defence, he is concerned only with the question of whether there is a case fit to be tried. As Dixon J. (as he then was) put it, the prosecutor must believe that ‘ the possibility of the accused’s guilt is such that upon general grounds of justice a charge against him is warranted [Commonweatlth Life Assurance Society Ltd. V. Brain (1935) 53 C.L. R. 343, at 382].