Why were mercantalist thinkers concerned with the acquisition of specie as opposed to overall productive capacity?

Mercantilist thinkers believed that a nation's wealth and power were directly tied to the amount of precious metals (specie) it possessed. They thought that a positive balance of trade, where a nation exported more than it imported, would lead to an influx of specie and increased prosperity. This focus on accumulating specie overshadowed the importance of overall productive capacity in their economic theories.