The senate again, which possesses such great power, is obliged in the first place to pay attention to the commons in public affairs and respect the wishes of the people,
and it cannot carry out inquiries into the most grave and important offences against the state, punishable with death, and their correction, unless the is confirmed by the people.
The same is the case in matters which directly affect the senate itself. For if anyone introduces a law meant to deprive the senate of some of its traditional authority, or to abolish the precedence and other distinctions of the senators or even to curtail them of their private fortunes, it is the people alone which has the power of passing or rejecting any such measure.
And what is most important is that if a single one of the tribunes interposes, the senate is unable to decide finally about any matter, and cannot even meet and hold sittings;
and here it is to be observed that the tribunes are always obliged to act as the people decree and to pay every attention to their wishes. Therefore for all these reasons the senate is afraid of the masses and must pay due attention to the popular will.
Walbank Commentary