The Senate by exercising a probouleutic authority showed its sense of its own limitations. Occasionally, as we shall see, it usurped isolated powers that belonged of right to the people; but as a rule its final authority was only felt in that vast sphere of executive influence that had been formerly entrusted to the magistrates. It could control, but it might not usurp, the sovereign powers of the people; it elected no magistrates; it possessed no legislative authority; it could not declare war or make peace; it dared not extend the limits of Roman citizenship by the conferment of the franchise; it made no claim to the exercise of jurisdiction or of the still more sovereign right of pardon. Yet, when it is remembered that the activity of the _comitia_ in all these matters could only be set in motion by the magistrate, and that the Senate’s advice had grown into a real power of control, it is easy to see that the first step in every measure of importance must come under the cognisance of this all-pervading council. Its probouleutic authority was based on the observance by the magistrate of certain unwritten rules, which regulated the exercise both of his positive and of his negative powers. It was held that no magistrate should question the people on any important matter without the Senate’s advice, and that he should not decline to exercise this power at its request; that the power of veto should be employed only at the discretion of the council, and that the request for its exercise should not be refused. These unwritten principles were, as a rule, strong enough to fetter a magistrate’s action by his conviction of their necessity; when this conviction was not sufficiently strong, the Senate resorted to its last constitutional weapon, the veto of a friendly magistrate. For this purpose the tribunician college was usually employed; its size offered the largest scope for differences of opinion, for of the ten legally supreme magistrates of the state one at least could generally be found to whom the Senate’s word was law. The blocking of legislation through the _intercessio_ of a senatorial tribune may be illustrated by the attempt of Octavius to negative the agrarian legislation of the elder Gracchus:[1327] the effort to annul an administrative order of a sweeping kind by the veto pronounced against the proposal of the praetor Juventius when, without consultation of the Senate and without previous intimation to the consuls, he attempted to urge the _comitia_ of the people into a declaration of war with Rhodes.[1328]