Significant challenges exist to the classical definition of "public goods", in general, that are also relevant to the definition of "global public goods". Kaul et al. (2003), suggest that there are actually three types of public goods. First, there are public goods that "cannot be made excludable", either because they are inherently indivisible or because the cost of division would be prohibitive. A simple example would be sunlight. Second, there are goods that are inherently "public by design". Examples include a nation's judiciary system or basic education system. A third type, they argue, are goods that are "public by default", either due to lack of foresight or knowledge in the design. An example of this type would be the ozone layer and damage done to the environment by chlorofluorocarbon (CFC) emissions before anyone understood the potential for damage.