The Wealth of Nations by Part 5 Chapter 1 Page 22

age, or a certain number of them, to join in some measure the trade of a soldier to whatever other trade or profession they may happen to carry on.

Or, secondly, by maintaining and employing a certain number of citizens in the constant practice of military exercises, it may render the trade of a soldier a particular trade, separate and distinct from all others.

If the state has recourse to the first of those two expedients, its military force is said to consist in a militia; if to the second, it is said to consist in a standing army. The practice of military exercises is the sole or principal occupation of the soldiers of a standing army, and the maintenance or pay which the state affords them is the principal and ordinary fund of their subsistence. The practice of military exercises is