OQ #15

Historian James Truslow Adams coined the phrase X in his 1931 book Epic of America:

The X is that ____ of a land in which life should be better and richer and fuller for every man, with opportunity for each according to ability or achievement. It is a difficult ____ for the European upper classes to interpret adequately, also too many of us ourselves have grown weary and mistrustful of it. It is not a ____ of motor cars and high wages merely, but a ____ of social order in which each man and each woman shall be able to attain to the fullest stature of which they are innately capable, and be recognized by others for what they are, regardless of the fortuitous circumstances of birth or position.

All blanks are the same. Identify the phrase.