we have two propositions identical in the second side of each we may substitute, getting
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:—
Taking our letters thus—
the premises are of the forms
| A = A b , | (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)
we can substitute in (1) obtaining the conclusion
But the same result may be obtained by taking the eight combinations of A, B, C, of the Logical Alphabet; it will be found that only