Part III, Chapter 3: General Rules of Simple Incomplex Syllogisms
Rule 3
No conclusion can be drawn from two negative propositions.
For two negative propositions separate the subject from the mean, and the attribute from the same mean. Now, because two things are separated from the same thing, it does not follow either that they are, or that they are not, the same; for because the Spaniards are not Turks, and the Turks are not Christians, it does not follow that the Spaniards are not Christians; neither does it follow that the Chinese are so, though they are not Turks any more than the Spaniards.