Historians divide the past into periods on the basis of economic and social factors to characterise the major elements of different moments of the past. Yes, the historians face a lot of problems because economic and social factors do not exist in a stasis, they have a tendency to change constantly.
Thomas Jefferson used the thoughts first penned by John Locke while writing the Declaration of Independence. The phrase "life, liberty, and pursuit of happiness," was an idea first considered by Locke in his Two Treatises on Government.
