of ecological (also biological) variables b which interact in their
dynamic t evolution: det dbt dt = f (et, bt)' dt = 9 (et, bt)- Among the
solution paths to this interaction between economic and ecologi- cal
variables, we look for those which are sustainable. Sustainable paths
are typically those along which the values of certain key stocks are
always pos- itive, these key stocks being important environmental
resources. The types of paths on which certain variables can be positive
forever include station- ary solutions with appropriate positivity
conditions, or limit cycles or chaotic attractors satisfying the same
positivity conditions. These paths, and the paths which approach them,
constitute the set of sustainable paths. From amongst these we have to
choose one or more which are in some sense the best. Note that rather
than imposing positivity of certain stocks in the long run as a
condition for sustainability, we would prefer to derive this as a
characteristic of optimal solutions from more fundamental judgements
about the valuation of stocks and flows: this is the route pursued by
the papers in this volume. The introductory paper by Heal in Section I
reviews these matters in gen- eral terms, not going into technical
details: it discusses the precedents for a concept of sustainability in
welfare economics, and reviews alternative opti- mality concepts and
their connection to sustainability.