strict liability and negligence liability

Posner explains the relationship between negligence and strict liability as follows:

The baseline common law regime of tort liability is negligence. When it is a workable regime, because the hazards of an activity can be avoided by being careful (which is to say, nonnegligent), there is no need to switch to strict liability. Sometimes, however, a particular type of accident cannot be prevented by taking care but can be avoided, or its consequences minimized, by shifting the activity in which the accident occurs to another locale, where the risk or harm of an accident will be less..., or by reducing the scale of the activity in order to minimize the number of accidents caused by it.... By making the actor strictly liable—by denying him in other words an excuse based on his inability to avoid accidents by being more careful—we give him an incentive, missing in a negligence regime, to experiment with methods of preventing accidents that involve not greater exertions of care, assumed to be futile, but instead relocating, changing, or reducing (perhaps to the vanishing point) the activity giving rise to the accident... The greater the risk of an accident...and the costs of an accident if one occurs..., the more we want the actor to consider the possibility of making accident-reducing activity changes; the stronger, therefore, is the case for strict liability.^[5]