Not necessarily. There are goods that present themselves with strong characteristics of "publicness", like knowledge. Still, it has been our choice to create intellectual property rights, which take some types of knowledge out of the public and into the private domain. The reverse also happens, as is the case for basic education. Individuals are often willing to pay in order to receive its benefits, and it is still largely private. However, current efforts to ensure "basic education for all" could place it in the public domain (see http://www.unesco.org/education/efa/index.shtml ). In some countries public education goes far beyond the basic level--it is provided publicly all the way up to higher education.

Therefore, notions of what is public and private change over time and differ from place to place. This alone is proof that public and private goods do not necessarily stay either public or private. The key point is that it is often a matter of policy choice to determine whether a good is either public or private. As the authors of Providing Global Public Goods put it, publicness and privateness are "social constructs". In fact, barring a few cases (such as sunlight, for example, which is difficult to take out of the public domain) we almost always have a choice on what to make public or private.