| Vowels are letters which can be sounded alone, | (1) |
| The letter w cannot be sounded alone; | (2) |
| Therefore the letter w is not a vowel. | (3) |
Here we have a definition (1), and a comparison of a thing with that definition (2), leading to exclusion of the thing from the class defined.
Taking the terms
A = vowel,
B = letter which can be sounded alone,
C = letter w,
the premises are plainly of the forms
| A = B, | (1) |
| C = bC. | (2) |
Now by the Indirect method we obtain from (1) the Contrapositive
b = a,
and inserting in (2) the equivalent for b we have
| C = aC, | (3) |