An injurer avoids liability in negligence when any one of the ingredients of negligence is absent. A potential injurer therefore has a number of possible ‘defences’, each one corresponding to the absence of an ingredient. Ex ante, a rational potential injurer will take care up to, but only up to, the point at which the cheapest available defence is acquired. We consider here the defences of absence of duty, absence of breach, absence of causation and absence of foreseeability, in the first place without uncertainty as to the legal standard. Absence of breach, causation and foreseeability are all related to the amount of care. Comparing the amounts of care necessary to acquire each defence, our principal result is that the defence of absence of causation is always cheaper to acquire than that of absence of breach, implying that a potential injurer will respond to a socially optimal legal standard by taking less than socially optimal care. When the model is extended to include uncertainty as to the legal standard the privately optimal amount of care increases and approximates to the social optimum.