Defense planning unavoidably must be in the nature of a mystery tour. The inability to know the future is a permanent condition for defense planning, but it is one with which we must cope. This monograph by Dr. Colin S. Gray explores and examines the implications of our irreducible ignorance about the future. His purpose is to identify an approach to this critically important subject of security that leans heavily upon what we can and should know about the past and present, in order to anticipate future conditions.The monograph finds that, although the past does not repeat itself in detail, there are profound persisting reasons why it is repeated approximately in the challenges and dangers that security communities must face. Dr. Gray concludes that notwithstanding the facts of contextual change in strategic history, the "great stream of time" from the past, through the present, into the future commands critically significant continuities in history that yield an approach to the future in which some confidence can be placed.The purpose of this monograph is to explore and examine the challenge to America's defense planners of needing to make purposeful and prudent choices in military preparation for the future. The problem for defense planning that is beyond resolution is the scientifically certain fact that we have no data from the future about the future. Moreover, this will always be a fact. No matter the scholarly discipline and tradition to which a defense planner owes or feels most allegiance, he or she needs to recognize and attempt to understand fully a personal and institutional condition of awesome ignorance of detail about the future.Further study, more cunning analytical methodology, even more powerful computers — none of these can reveal with any certainty what the future will bring. Fortunately, this does not mean that we are ignorant about the future; but it does mean that defense planning is guesswork and can only be such. Understandably, both senior policymakers and soldiers tend to be reluctant, even to the point of appearing to be evasive, when legislators question the plausibility of the answers given in congressional hearings. After all, it can be troubling to the conscience of honest and competent people to be obliged to affirm the integrity of choices made in defense preparation for national security in years to come, when there is and can be no certain way to know that one is sufficiently correct.Although the future is always a tabula rasa concerning the detail, including vital detail, of what will happen, the human security condition is anything but unknown, let alone unknowable. A key to making progress here is to pose only answerable questions. For a leading example of a foolish question, one should never ask "What will happen?" Reliable answers cannot be given with the certainty required of science.