The method of superimposition is virtually employed in all competitive struggle. The glory and fruits of victory are sought by both opponents, and the success of one is the failure of the other. The superiority of the victor to the vanquished is beyond question only because they had the same interest at stake. The application of this method to the determination of progress is not confined to philosophers of history. It is applied by every individual who realizes that his advance from childhood to maturity has been attended with growth and development. For the old boundaries of childhood still remain as evidence of the greater magnitude of the life which has outgrown them. Similarly every man may mark within himself the various limits which once bounded him, but which he has since exceeded in consequence of steady and consecutive effort. The progress of mankind at large differs only in complexity and range. It can be tested and determined only because identical interests persist. If men had not in all times wanted the same things it would be impossible to measure their attainments. Their successes and failures would be incommensurable. But the old needs and the old hopes yet remain. The problem of life which was from {130} the beginning is a problem still. If it can be shown that the old needs are met more easily, along with new needs besides, that there is better promise that the hopes will be fulfilled, and that the general problem of life is nearer a solution, then human progress will have been demonstrated. II I propose, in the first place, to discuss two general principles, the operation of which is conducive to progress. One of these principles is _external_, that is, it relates to the environment of life rather than to its internal economy; and to this I shall turn first. The external environment of life is in some respects favorable, in other respects unfavorable. Now, strangely enough, it is the unfavorable rather than the favorable aspect of the environment that conduces to progress. Progress, or even the least good, would, of course, be impossible, unless the mechanical environment was morally plastic. The fact that nature submits to the organization which we call life is a fundamental and constant condition of all civilization. But there is nothing in the mere compliance of nature to press life forward. It is the _menace_ of nature which stimulates progress. It is because nature always remains a source of difficulty and danger {131} that life is provoked to renew the war and achieve a more thorough conquest. Nature will not permit life to keep what it has unless it gains more.