 Another possible route to knowledge of the boundary locations is blocked by the fact that our knowledge of the application of a vague term is inexact. Inexact knowledge is governed by margin for error principles, viz., principles of the form ‘If x and y differ incrementally on a decisive dimension and x is known to be \(\Phi\) (old, blue, etc.), then y is  \(\Phi\)’.[2]  For example, where knowledge is inexact, we can know of a blue object that it is blue only if objects whose colors are incrementally different are blue as well—hence, only in clear cases. In contrast, in the borderline or “penumbral” region of a sorites series for ‘blue’, where the boundary lives, some shade of blue is only incrementally different from, indeed may look the same as, a shade that is not blue; and we cannot know where this difference lies. Consequently, if we classify the former shade as blue, that classification is correct by luck, and so does not constitute knowledge. (On the plausible assumption that seeing that something x is blue is sufficient for knowing that x is blue, it follows that some blue things are such that we cannot see that they are blue, even under ideal viewing conditions.)