Aristotle's Ethics by Aristotle

Ambition and lack of ambition are the extremes of a nameless virtue which is related to high-mindedness in the same way that generosity is related to magnificence. It may be defined as desiring honor in the right amount, in the right manner, for the right reason, and at the right time. The ambitious man strives for more honor in any particular area than he is entitled to. The unambitious man tries to avoid honor, even for noble achievements, from a false sense of modesty or because he does not properly judge his own worth. This virtue resembles lack of ambition when compared to ambition, and ambition when compared to lack of ambition, but from a distance it can look like both at once.