According to Fischer and Newell, "if learning is more firm-specific and less likely to spill over, policies subsidising renewables are less appropriate to compensate for knowledge externalities. In contrast, if learning is more difficult to patent to appropriate rents, then renewable subsidies may be relatively more justified" than R&D support. A more modern - and possibly more realistic - perspective instead sees technical change as a cyclical process, based on two-way feedbacks between market experiences and technical developments.