David Lewis holds that the standard of evidence required for knowledge varies with context. This theory, contextualism, provides a partial, but only a partial, response to external world scepticism.

In the context of everyday life, the standard of evidence required for knowledge is relatively low. This means that we don’t have to exclude sceptical hypotheses such as the evil demon hypothesis in orer to have knowledge in the everyday sense.

In the context of a philosophical discussion of scepticism, the standard of evidence required for knowledge is very high. Once sceptical hypotheses have been raised, if they cannot be refuted then we cannot have knowledge.

This means that a student who arrives at a philosophy class on scepticism knows the contents of her bag when she arrives. When the lesson begins, however, and the context changes, she loses this knowledge, only to regain it on exiting the class.

As long as we don’t think about scepticism, we have knowledge.