This is because transaction costs are a residual category that includes all those effects that economists are unable to define properly. Going a step further, one can identify two major sub-categories of transaction costs that can help to understand the non-existence of markets and the existence of external costs. The absence of property rights is due to either the physical nature of the good (such as clean air) or due to ethically motivated decisions (such as maintenance of beaches as common properties). Obviously, the costs and difficulties involved in defining property rights can be defined as a transactions cost.