 Recently some philosophers have thought that a person can be justified in believing, or accepting, a proposition without having any evidence to support it, and so even if there is no nonexperiential source of evidence for that belief or acceptance. As we have seen, Timothy Williamson has argued that certain acquired skills can be used to provide justification for believing a proposition for which the person does not have evidence, namely, the skill of bringing ideas together in imagination. His proposal is that in the case of the inches/centimeters proposition it is this skill at making comparisons of length in imagination, and then observing the results, not a person’s perceptual evidence, that justifies him in believing that proposition.