The particular aspect of ...

The particular aspect of history which both attracts and benefits its readers is the examination of causes and the capacity, which is the reward of this study, to decide in each case the best policy to follow. Now in all political situations we must understand that the principle factor which makes for success or failure is the form of a state's constitution: it is from this source, as if from a fountainhead, that all designs and plans of action not only originate but reach their fulfillment.
 Polybius

Quotes from the same author

Nor ought we ever to allow any growing power to acquire such a degree of strength as to be able to tear from us, without resistance, our natural, undisputed rights.
 Polybius
The mob is easily led and may be moved by the smallest force, so that its agitations have a wonderful resemblance to those of the sea.
 Polybius
From this I conclude that the best education for the situations of actual life consists of the experience we acquire from the study of serious history. For it is history alone which without causing us harm enables us to judge what is the best course in any situation or circumstance.
 Polybius
Monarchy degenerates into tyranny, aristocracy into oligarchy, and democracy into savage violence and chaos.
 Polybius
The glorious memory of brave men is continually renewed; the fame of those who have performed any noble deed is never allowed to die; and the renown of those who have done good service to their country becomes a matter of common knowledge to the multitude, and part of the heritage of posterity.
 Polybius