In neoclassical theory, knowledge generates increasing returns – and therefore growth – because it is a public good that can be costlessly reused once created. In fact, however, much knowledge in the economy is actually tacit and not easily transmitted –and thus not an obvious source of increasing returns. Several writers have responded to this alarming circumstances by affirming hopefully that knowledge today is increasingly codified, general, and abstract – and increasingly less tacit. This paper disputes such a trend. But all is not lost: for knowledge does not have to be codified to be reused and therefore to generate economic growth.