Besides the general public interests of the state there are private and personal interests and ruling passions in princes and in their ministers or favourites, which often play a determining part in the direction of public policy. It is therefore necessary for the negotiator to inform himself of the nature of these private interests and passions influencing the spirits of those with whom he has to negotiate, in order that he may guide his action by this knowledge either in flattering their passions, which is the easiest way, or by somehow finding means to deflect such personages from their original intentions and engagements and cause them to adopt a new line of policy. Such an enterprise carried to success would indeed be a masterpiece of negotiation. [Sidenote: _Testimony of the Duc de Rohan._] That great man, the Duc de Rohan, tells us in the treatise which he wrote upon the interests of European sovereigns, that the sovereigns rule the people and that interest rules the sovereign; but we may add that the passions of princes and of their ministers often overrule their interests. We have seen many cases in which monarchs have entered engagements most prejudicial to themselves and their state under the influence of passion. There need be no surprise on this account, for the nations themselves are not free from this error, and are prepared to ruin themselves in order to satisfy hatred, vengeance, and jealousy, the satisfaction of which is often antagonistic to their veritable interests. Without recourse to ancient history it would be easy to prove by modern examples that men do not act upon firm and stable maxims of conduct; that as a rule they are governed by passion and temperament more than by reason. The bearing of this knowledge upon diplomacy is that since the passion and caprice of men in authority so largely influence the destiny of their subjects, it is the duty of the able negotiator to inform himself as accurately as possible regarding the inclination, state of mind, and the plans of men in authority in order that this information may be placed at the service of his master’s interests. And we may be sure that a negotiator who has not laboured to acquire a fund of this general and particular information will reason falsely regarding events, affairs of state, and men, and is liable to make false estimates and give dangerous advice to the prince who employs him. Such knowledge is not to be found in books alone; it is more easily to be gathered by personal communication with those engaged in public service and by foreign travel, for, however profoundly one may have studied the customs, the policy, or the passions of those who govern in foreign states, everything will appear differently when examined close at hand, and it is impossible to form a just notion of the true character of things except by first-hand acquaintance. [Sidenote: _Importance of Foreign Travel._]