there was nothing could be said to that argument , so servius changed the subject , encouraging the men to imagine how the walls would look to an approaching enemy , how the northern celts for instance , or the southern greeks , might have cast envious eyes on the growing wealth of the city , but this would deter them ; but he knew , all the time , the men had only half a mind on what he was saying , but were still thinking about their farms , their businesses , the way rome had stolen their time and given them only mud and backache to show for it .