From the point of view of the conduct of war, therefore, we have two well-defined categories of blockade, naval and commercial. But our classification must go further; for naval blockade itself is equally varied in intention, and must be subdivided. Strictly speaking, the term implies a desire to close the blockaded port and to prevent the enemy putting to sea. But this was not always the intention. As often as not our wish was that he should put to sea that we might bring him to action, and in order to do this, before he could effect his purpose, we had to watch the port with a fleet more or less closely. For this operation there was no special name. Widely as it differed in object from the other, it was also usually called blockade, and Nelson's protest against the consequent confusion of thought is well known. "It is not my intention," he said, "to close-watch Toulon"; and again, "My system is the very contrary of blockading. Every opportunity has been offered the enemy to put to sea." It is desirable, therefore, to adopt terms to distinguish the two forms. "Close" and "open" express the antithesis suggested by Nelson's letter, and the two terms serve well enough to mark the characteristic feature of each operation. Close blockade, it is true, as formerly conceived, is generally regarded as no longer practicable; but the antithetical ideas, which the two forms of blockade connote, can never be eliminated from strategical consideration. It must always be with the relations of these two forms, whatever shape they may take in future, that the strategy of naval blockade is chiefly concerned. With regard to commercial blockade, in strict analysis it should be eliminated from an inquiry that concerns methods of securing command and postponed to that section of exercising command which deals with the attack and defence of trade. It is, however, necessary to treat certain of its aspects in conjunction with naval blockade for two reasons: one, that as a rule naval blockade is indissolubly united to a subordinate commercial blockade; and the other, that the commercial form, though its immediate object is the exercise of control, has almost invariably an ulterior object which is concerned with securing control; that is to say, while its immediate object was to keep the enemy's commercial ports closed, its ulterior object was to force his fleet to sea.