 Another, unfortunately obscure canon identifies four sources of doubt, or potential causes of erroneous judgment (B10): accidental circumstances, inconclusive evidence, causal overdetermination, and transience. The first of these seems to refer to error due to atypical circumstances. In a dense fog, for instance, someone might mistake a man for an ox. Unseasonable weather might cause a person who has moved to a light hut in summer to unexpectedly feel cold. The possibility of error in such difficult or unexpected circumstances can provide a reason to doubt a judgment. The second source of doubt is insufficient evidence. We have reason to doubt that someone who lifts a load of feathers is strong or that someone who shaves wood along the grain is skilled woodworker, because these tasks are too easy to be firm evidence of strength or skill. The third source is the presence of multiple causes that could explain something. The cause of a fighter’s collapse might have been that he was drinking alcohol before the match or that he was overcome by the heat of the noonday sun. Since we cannot know which factor was decisive, we have grounds to doubt any claim that one or the other was. The fourth source refers to situations in which the object of knowledge may change, such that what we know ceases to be the case. An obvious example—the Mohists do not give one—is knowing the status of the weather. We knew it was raining when we ran in from the storm, but after some time we may have grounds to doubt whether it is still raining.