The above three principles imply yet another: the rule that has come to be known as the "leave enough behind" rule, or "Locke's Proviso." According to this proviso, an individual can legitimately create personal property interests in the bounties of nature only so long as "there is enough and as good left in common for others." f80 Locke applied this rule to the appropriation of land, water, animals, and the fruits of the earth. f81
The Proviso was probably articulated with intragenerational concerns in mind, more than intergenerational ones. Writing in the 17th century, Locke could not foresee how the overutilization of natural resources by one or more generations would prejudice members of later generations. As he noted in paragraph 36 of the Second Treatise, "there is land enough in the world to suffice double the inhabitants." f82 Over the intervening centuries, however, the world's population has doubled many times over, and Locke's basic principles of just resource appropriation have implications today that were not readily apparent when he first formulated them.
Extending the Proviso's coverage to include future generations gives due effect to the underlying principles of equity upon which the Proviso is based. f83 To require that a person appropriating property leave "enough and as good" for the earth's future tenants as well as its present tenants is to honor Locke's emphasis on the preservation of the species along with his view of the Earth as an intergenerational commons. Read in this way, the Proviso clearly prohibits a variety of unsustainable practices -- practices which result, individually or cumulatively, in the extinction of species, and the depletion or spoilage of natural resources. Because such practices do not leave "enough and as good" for later tenants of the intergenerational commons, the right to engage in such practices is not a right which can ever be legitimately created or conveyed. f84
It is important to recognize that when Locke discussed the above four intergenerational principles, he did not limit their application to some semi-mythical "state of nature." The rules were held to be applicable to citizens living and working within political societies as well. The laws of a commonwealth, he maintained:
"[are] only so far right as they are founded on the law of nature, by which they are to be regulated and interpreted. . . . The rules that [legislators] make for other mens' actions must . . . be conformable to the law of nature, i.e., to the will of God, of which that is a declaration . . . [t]he fundamental law of nature being the preservation of mankind . . . The end and measure of the [political] power is the preservation of all . . . society -- that is, all mankind in general." f85