[In science any model depends on a pre-chosen taxonomy] a set of classifications into which we divide the enormous complexity of the real world... Land, labor, and capital are extremely heterogeneous aggregates, not much better than earth, air, fire, and water.

We should always bear in mind that numbers represent a simplification of reality.

The social dynamics of human history, even more than that of biological evolution, illustrate the fundamental principle of ecological evolution - that everything depends on everything else. The nine elements that we have described in societal evolution of the three families of phenotypes - the phyla of things, organizations and people, the genetic bases in knowledge operating through energy and materials to produce phenotypes, and the three bonding relations of threat, integration and exchange - all interact on each other.

In 1859 the human race discovered a huge treasure chest in its basement. This was oil and gas, a fantastically cheap and easily available source of energy. We did, or at least some of us did, what anybody does who discovers a treasure in the basement - live it up, and we have been spending this treasure with great enjoyment

The future is bound to surprise us, but we don't have to be dumbfounded.

The ability to work with systems of general equilibrium is perhaps one of the most important skills of the economist - a skill which he shares with many other scientists, but in which he has perhaps a certain comparative advantage.

[The loss-of-strength gradient is] the degree to which military and political power diminishes as we move a unit distance away from its home base.