[填空题]There is much discussion in the literature these days about how one "defines" sustainability, sustainable development, and related concepts. Many argue that the concept is useless because it cannot be "adequately defined". 46) Most of this discussion is misdirected because it attempts to cast the problem as definitional, when in fact it is a problem of prediction, and it fails to take into account the many time and space scales over which the concept of sustainability must apply. Defining sustainability is actually quite easy: A sustainable system is one that survives for some specified (finite) time. The problem is that one knows one has a sustainable system only after the fact. Thus, what usually pass for definitions of sustainability are actually predictions of what set of conditions will actually lead to a sustainable system. 　　47) For example, keeping harvest rates of potentially renewable resources such as trees and wildlife below rates of natural renewal should, one could argue