Organ economics.

[An orthodox economist] would find it inefficient to carry two lungs and two kidneys[…]. [Nature] would dispense with individual kidneys – since we do not need them all the time, it would be more ‘efficient’ if we sold ours and used a central kidney on a time-share basis. You could also lend your eyes at night, since you do not need them to dream. (128)

Nassim Nicholas Taleb, qtd. in Nikiforuk, Andrew. The Energy of Slaves: Oil and the New Servitude. Madeira Park, BC: Douglas & McIntyre, 2012. EPUB file.