 Even if true, it seems that we can distinguish empirically based from understanding based employment of a skill. Manipulating in imagination two circles of unequal radii can bring a person to understand that they cannot intersect in more than two points. Creating an image of a line nine inches long next to an image of a typical ant can bring a person to believe that the distance between the front and back legs of a typical ant is less than nine inches, but not that it must be. Science fiction films sometimes depict giant ants that could have been actual. Further, skills at judging, say, that in Sheep you do not know that there are sheep in the field is based on your understanding the concept “knowledge”. But your skill in judging that sheep have wool requires more to qualify as knowledge than understanding that proposition: you need to know what sheep in fact are like. A detective’s skill at deducing via disjunctive syllogism that Jones is the murderer is similar to a geometer’s deducing that a quadrilateral has four interior angles or that the Pythagorean Theorem is true, but the nature of the premises can make all the difference between whether the reasoning is a posteriori or a priori. So even if the exercise of relevant skills can provide justification apart from evidence, how the skill is used, and in particular on what subject matter, seems to be a basis for distinguishing a priori from a posteriori (empirical) justification.