Deterrence is the cardinal concept of nuclear strategy, and consequently we have to examine its basic determinants. An analysis of this sort should at the outset be abstract and general, equivalent to a discussion of the 'grammar' of the subject. Deterrence contains the following elements: deterrer, deterred, threat, commitment, and persuasion. The credibility of a threat will be impaired if the deterred concludes that the deterrer is either not capable of executing his threat, or would at the crucial moment hesitate to implement it, and retreat. Capability means the threatener's ability to implement his threat. The debate on invulnerability gave birth to a distinction between first-strike capability or first-strike forces, designed for the initiation of nuclear war, and second-strike capability, meaning forces surviving an enemy attack. Concealment and secrecy–emplacing weapons in locations unknown to the enemy and camouflaging them. The threat to employ a doomsday machine is an extreme example of the rationality of irrationality.