A = some
B = nebulæ
C = giving continuous spectra
D = heated solids
The premises then become
| D | = DC | (1) |
| AB | = ABc | (2) |
Now from (1) we obtain by the indirect method the contrapositive proposition
c = cd
and if we substitute this expression for c in (2) we have
AB = ABcd
the full meaning of which is that “some nebulæ do not give continuous spectra and are not heated solids.”
We might similarly apply the contrapositive in many other instances. Take the argument, “All fixed stars are self-luminous; but some of the heavenly bodies are not self-luminous, and are therefore not fixed stars.” Taking our terms
A = fixed stars
B = self-luminous
C = some
D = heavenly bodies