'The man without one and the same aim in life cannot himself stay one and the same throughout his life.' The maxim is incomplete unless you add what sort of aim that should be. Judgements vary of the whole range of various things taken by the majority to be goods in one way or another, but only one category commands a universal judgement, and that is the good of the community. It follows that the aim we should set ourselves is a social aim, the benefit of our fellow citizens. A man directing all his own impulses to this end will be consistent in all his actions, and therefore the same man throughout.
The hill mouse and the house mouse - and the frightened scurrying of the house mouse.
The aim (goal) of all our actions is social. Some people will think the purpose and aim of life is to live a life of pleasure and ease and avoidance of pain. Some people may think the purpose is to "win" at everything - to win the most possessions, to win the biggest promotion, or to be a champion. But all these ring hallow. The best aim in life is to help and serve others (a social aim) - to benefit our fellow citizens. Some may spend their days feeding the hungry, some may teach children and students. Some may govern and others may work to provide good infrastructure to life the quality of life. However we decide to spend our time, be mindful of the purpose.
Marcus Aurelius spoke of two mice - the one from the city and the one from the country. The house mouse, who lives in the city, scurries about trying not to die from the home-owners, while also trying to live off scraps of food. The hill mouse has less worries, may find food - may have to work a bit for it - but does not worry about residents trying to kill him. Indeed the metaphor isn't spot on, as the hill mouse may have to worry about predators. But the principal is about priorities.
(see also Citadel p. 301)