Two sentences are logically equivalent if they are always both true or both false. Using this definition, we can consider three operations on categorical sentences. Conversion of E and I sentences results in a logically equivalent sentence. Conversion of A and O sentences tells us nothing. Contraposition of E and I sentences tells us nothing.
All humans are mortals.
All mortals are organisms. Therefore, all humans are organisms. We can substitute letters for the categories. Then we construct the venn diagram, and compare it to the venn diagram for the conclusion. Another valid syllogism Some students are persons who went to Florida last year.