Williamson has contributed to analytic philosophy of language, logic, metaphysics and epistemology.

In actuality, this condition will be spelled out only partly in terms of numbers of hairs, but whatever measures are relevant will have some sharp cutoff.

This solution to the difficult Sorites paradox was considered an astonishing and unacceptable consequence, but has become a relatively mainstream view since his defence of it.

This went against the common trend in philosophical literature up to that point, which was to argue that knowledge could be analysed into constituent concepts.

He agrees that knowledge entails justification, truth and belief, but argues that it is conceptually primitive.

However, Williamson has also developed an ontology of bare possibilia which he argues alleviates the worst consequences of necessitism and of the Barcan formula.