political economy vs economics

In "Global political economy: understanding the international economic order" (2001), Robert Gilpin writes (p25-6):

'For Adam Smith, political economy was a "branch of the science of a statesman or legislator", or as John Stuart Mill, the last major classical economist commented, political economy was the science that teaches a nation how to become rich. These thinkers emphasised the "wealth of nations", and the term "political" was as significant as the term "economy".'

'In the late 19th Century, [Gilpin continues], this broad definition of what economists study was narrowed considerably. Alfred Marshall, the father of modern economics, ...substituted the present-day term "economics" for "political economy" and greatly restricted the domain of economic science. '

For Marshall, Economics was a 'value-free, empirical science'. In the modern terminology of Economics today, continues Gilpin, it is 'defined by economists as a universal science of decision-making under conditions of constraint and scarcity' .