Adam Smith recognised that an invisible hand guided a market towards an equilibrium in which the buyer and seller would gradually refine their deals until they couldn’t improve upon it. Smith’s other idea, the division of labour, ensures that our returns are perenially increasing. As each member in a supply chain specialises and improves upon their productivity, the total cost of any product decreases. Innovations occur at this level, inventing new tools, machinery, materials, designs, and techniques. In 1942, Joseph Schumpeter argued that innovation had the most profound impact on the growth of an economy. In some estimates, further investment in production makes up just 15% of economic growth, with the remaining 85% attributable to innovation. In 1990, Paul Romer recognised that innovation was an output, as well as an input. It was non-rival, meaning infinite sharability. But it was also partially excludable - those who first acquired it could profit from the discovery. Knowledge is both a public good and a temporarily private one. It is expensive to produce but can occasionally more than pay for itself.