What is the test for the offence of conspiracy?

The offence of conspiracy involves proof of an agreement between two or more persons to commit an indictable offence and proof of a common intention to pursue the unlawful object. These two elements, the actus reus and the mens rea of the offence, are described by Tashereau J. in Regina v. O'Brien 1954 CanLII 42 (SCC), [1954] 110 C.C.C. 1 (S.C.C.) in the following passages at p.p. 3-4: The two elements of agreement and of common design are specifically stated to be essential ingredients of the crime of conspiracy. ... A conspiracy consists not merely in the intention of two or more, but in the agreement of two or more to do an unlawful act, or to do a lawful act by unlawful means. So long as such a design rests in intention only, it is not indictable. When two agree to carry it into effect, the very plot is an act in itself, and the act of each of the parties..... punishable if for a criminal object." ... The definition of conspiracy presupposes an aim. People do not conspire unless they have an object in view. The law punishes conspiracy so that the unlawful object is not attained. It considers that several persons who agree together to commit an unlawful act, are a menace to society, and even if they do nothing in furtherance of their common design, the state intervenes to exercise a repressive action, so that the intention is not materialized, and does not become harmful to anyone. The intention must necessarily be present because it is the unlawful act necessarily flowing from the intention, that the state wishes to prevent.