we have the premises
| A | = AB, | (1) |
| CD | = bCD | (2) |
Now from (1) we can draw the contrapositive
b = ab
and substituting this expression for b in (2) we obtain
CD = abCD
which expresses the conclusion of the argument that some heavenly bodies are not fixed stars.
Contrapositive of a Simple Identity.
The reader should carefully note that when we apply the process of Indirect Inference to a simple identity of the form
A = B