David Hume

Another contributor to early economic thought was David Hume, philosopher and good friend of Adam Smith. Hume's two most important and lasting contributions to economic thought are the price specie-flow mechanism and the neutrality of money. His main attack was on the Mercantilist belief that wealth could be maximized via specie accumulation(for which they advocated a balance of trade surplus). Hume argued that, under a gold standard system, a balance of trade surplus (or deficit) is self-liquidating. A surplus increases the country's specie, thus increasing the country's domestic money supply. Under an assumption of constant velocity of money, this increase in the money supply increases aggregate demand and thus, the price level. Higher domestic prices mean that the price of exports rise in relation to the price of imports. This price differentiation leads to increased imports and decreased exports until the surplus is liquidated. Neutrality of money refers to the fact that changes in the money supply do not affect real measures of the economy, such as real output and real employment, but rather exclusively affect nominal measures such as prices and wages.
Hume also influenced the ideas of Adam Smith, as they were close friends and colleagues.