Praeclarum Theorema/Historical Note

Proof
The praeclarum theorema was noted and named by, who stated and proved it in the following manner:


 * If $a$ is $b$ and $d$ is $c$, then $ad$ will be $bc$.
 * This is a fine theorem, which is proved in this way:
 * $a$ is $b$, therefore $ad$ is $bd$ (by what precedes),
 * $d$ is $c$, therefore $bd$ is $bc$ (again by what precedes),
 * $ad$ is $bd$, and $bd$ is $bc$, therefore $ad$ is $bc$.
 * Q.E.D.


 * (Leibniz, Logical Papers, p. 41).