There is no explaining how this idea came.

No more can it be explained than can a human explain why, at luncheon to-day, he selects green peas and rejects string beans, when only yesterday he elected to choose string beans and to reject green peas.

No more can it be explained than can a human judge, sentencing a convicted criminal and imposing eight years imprisonment instead of the five or nine years that also at the same time floated upward in his brain, explain why he categorically determined on eight years as the just, adequate punishment.
Since not even humans, who are almost half-gods, can fathom the mystery of the genesis of ideas and the dictates of choice, appearing in their consciousness as ideas, it is not to be expected of a more dog to know the why of the ideas that animate it to definite acts toward definite ends.
And so Jerry.

Just as he must immediately howl, he was aware that the idea, an entirely different idea, was there, in the innermost centre of the quick-thinkingness of him, with all its compulsion.

He obeyed the idea as a marionette obeys the strings, and started forthwith down the deck aft in quest of the mate.
He had an appeal to make to Borckman.