or what is not three-sided is not a triangle (whether it be rectilinear or not).
Second Example.
Let us treat by this method the following argument:—
“Blende is not an elementary substance; elementary substances are those which are undecomposable; blende, therefore, is decomposable.”
Taking our letters thus—
A = blende,
B = elementary substance,
C = undecomposable,
the premises are of the forms
| A = Ab, | (1) |
| B = C. | (2) |
No immediate substitution can be made; but if we take the contrapositive of (2) (see p. [86]), namely
| b = c, | (3) |