Again, he has thought out the theory of taxation. He admits its necessity. The State is obliged to perform certain duties for the community. It is obliged, for example, to make its roads fit for travelling, and so render them passable for the transfer of merchandise. It is bound to clear away all brigandage, highway robbery, and the like, for were this not done, no merchant would venture out through that State's territory, and its people would accordingly suffer. Hence, again, he deduces the need for some sort of army, so that the goods of the citizens may be secured against the invader, for without this security there would be no stimulus to trade. Bridges must be built, and fords kept in repair. Since, therefore, the State is obliged to incur expenses in order to attain these objects, the State has the right, and indeed the duty, to order it so that the community shall pay for the benefits which it is to receive. Hence follows taxation. But he sees at once that this power of demanding forced contributions from wealthy members of society needs safeguarding against abuse. Thus he is careful to insist that taxation can be valid only when it is levied by public authority, else it becomes sheer brigandage. No less is it to be reprobated when ordered indeed by public authority, but not used for public benefit. Thus, should it happen that a prince or other ruler of a State extorted money from his subjects on pretence of keeping the roads in good order, or similar works for the advantage of the community, and yet neglected to put the contributions of his people to this use, he would be defrauding the public, and guilty of treason against his country. So, too, to lay heavier burdens on his subjects than they could bear, or to graduate the scale in such a fashion as to weigh more heavily on one class than another, would be, in the ruler, an aggravated form of theft. Taxation must therefore be decreed by public authority, and be arranged according to some reasonable measure, and rest on the motive of benefiting the social organisation. The citizens therefore who are elected to settle the incidence of taxation must be careful to take account of the income of each man, and so manage that on no one should the burden be too oppressive. He suggests himself the percentage of one pound per hundred. Nor, again, must there be any deliberate attempt to penalise political opponents, or to make use of taxation in order to avenge class-oppression. Were this to be done, the citizens so acting would be bound to make restitution to the persons whom they had thus injured.