This happens in Sub-altem (A + I or E + O) consider the follow: All Trees are flowers (A) Some Trees are flowers (I) 'Some Trees are flowers' is true, it is difficult to come up with a single venn diagram (because there is a possibility that some trees are not flowers) In that situation, the universal positive (A) [All trees are flowers] will be undetermined because we can't draw a single venn diagram.