But what kind of power?
There are two large classes into which power may be divided, passive and active.

Certainly we seem justified, at the start, in declaring that the power meant by Mahan was not passive, but active.

Should this be granted, we cannot be far from right if we go a step further, and declare that sea power means ability to do something on the sea.
If we ask what the something is that sea power has ability to do, we at once perceive that sea power may be divided into two parts, commercial power and naval power.
The power exerted by commercial sea power is clearly that exerted by the merchant service, and is mainly the power of acquiring money.

It is true that the merchant service has the power of rendering certain services in war, especially the power of providing auxiliary vessels, and of furnishing men accustomed to the sea; but as time goes on the power contributable by the merchant service must steadily decrease, because of the relatively increasing power of the naval service, and the rapidly increasing difference between the characteristics of ships and men suitable for the merchant service and those suitable for the naval service.
But even in the past, while the importance of the merchant service was considerable in the ways just outlined, it may perhaps be questioned whether it formed an element of _sea power_, in the sense in which Mahan discussed sea power.

The power of every country depends on all the sources of its wealth: on its agriculture, on its manufacturing activities, and even more directly on the money derived from exports.
But these sources of wealth and all sources of wealth, including the merchant service, can hardly be said to be elements of power themselves, but rather to be elements for whose protection power is required.
In fact, apart from its usefulness in furnishing auxiliaries, it seems certain that the merchant service has been an element of _weakness_.