Frederick the Great of Prussia was a contemporary of Vattel, and in November 1760 he published some military instructions for the use of his generals which, in the matter of spies, was based on a wider practical knowledge of the matter than of course belonged to the more pacific publicist. He classified spies into ordinary spies, double spies, spies of distinction, and spies by compulsion. By double spies he meant spies who also pretended to be in the service of the side they betrayed. By spies of distinction he meant officers of hussars, whose services he had found useful under the peculiar circumstances of the Austrian campaign. When he could not procure himself spies among the Austrians, owing to the careful guard which their light troops kept round their camp, the idea occurred to him, and he acted on it with success, of utilising the suspension of arms that was customary after a skirmish between hussars to make those officers the means of conducting an epistolary correspondence with the officers on the other side. Spies by compulsion he explained in this way: ‘When you wish to convey false information to an enemy, you take a trustworthy soldier and compel him to pass to the enemy’s camp to report there all that you wish the enemy to believe; you also send by him letters to excite the troops to desertion.’ And in the event of its being impossible to obtain information about the enemy, this distinguished child of Mars prescribes the following: Choose some rich citizen, who has land and wife and children, and another man disguised as his servant or coachman, who understands the enemy’s language. Force the former to take the latter with him to the enemy’s camp to complain of injuries sustained, threatening him that if he fail to bring the man back with him after having stayed long enough for the desired object, his wife and children shall be hanged and his house burnt. ‘I was myself constrained,’ adds this great warrior, ‘to have recourse to this method, when we were encamped at ----, and it succeeded.’[193] Such were the military ethics of the great philosopher and king, whose character in the closer intimacy of biography proved so disagreeable a revelation to Carlyle. Pagan antiquity might be searched in vain for practice or sentiments more ignoble. Sertorius, the Roman captain, was one of the greatest masters of stratagem in the world, yet how different his language from that of the Great Frederick! ‘A man,’ he said, ‘who has any dignity of feeling should conquer with honour, and not use any base means even to save his life.’