From the earliest time, when people entered into contractual arrangements, many things could happen that would go wrong. When you're dealing with consensual arrangements, like medical surgery, everybody knows the moment you cut into somebody, you create a surgical wound. And so the notion of a strict liability system is utterly unattainable under these circumstances. So what happens is, instead you say that you're, as a doctor, free of liability unless you deviated from the customary standard of care, applicable to your particular specialty, and then you are only liable if the deviation was the cause of the injury in question. As time goes on, there are other cases of consensual arrangements that have to be dealt with. There are cases where people invite other people into their property. Those are called “occupier liability” cases and generally speaking if you invite somebody into your home and there's a latent defect so they fall down, you might be well responsible for the injury. Later on, we start talking about product liability cases. These arise when A sells goods to B, and then B sells them down to C, so there's no direct contract relationship between A and C, and the early law of product liability said if you create a trap or a latent defect in the thing that you sold, even in the absence of privity you could be responsible for the harm that was caused. Note that in the consensual cases the easiest line of defense is one of self-help: you don't have to buy somebody's products, you don't have to enter into their home, you can pick another surgeon. So that the consensual arrangements are more concerned with making contract arrangements more efficient and less concerned with the maintenance of social order.