In a Bayesian model of learning, the more an agent uses a technology, the better he learns its parameters. This expertise is a form of human capital. Switching to a new technology temporarily reduces expertise: the bigger the leap, the bigger the loss. This may prevent the agent from climbing...

This paper endogenizes the frequency of major discoveries and the extent of their refinement. Four axioms deliver a one-parameter family of beliefs that guide exploratory effort. Such effort trades off the prospect of major new discovery against the chance of successfully refining discoveries...