Conventional economic theory has to ignore energy or that would prove that there are Limits to Growth. An issue largely overlooked within conventional economic theory is that all economic systems are fundamentally physical systems in which energy is transmitted and converted into different forms, in compliance with the second law of thermodynamics—and in which depletion must take place with respect to non-renewable energy resources due to the conservation of matter and energy. Economic theory does not recognize that the laws of physics provide any meaningful constraint on the capacity of economies to grow continuously by forever increasing their material throughput. Rather, as with any complex system, the macro-structures emerge from but still operate within those laws. Thus, the direct correlation between economic growth and the growth of energy consumption is because economic growth is fundamentally dependent on and enabled by energy.