(1) A syllogism must have three, and only three, different terms.
(2) A syllogism must have three, and only three, propositions.
(3) The middle term must be distributed at least once.
(4) No term must be distributed in the conclusion which is not also distributed in a premise.
(5) No conclusion can be drawn from two negative premises.
(6) If one premise be negative, the conclusion must be negative; and conversely, to prove a negative conclusion, one of the premises must be negative.
(7) No conclusion can be drawn from two particular premises.
(8) If one premise be particular, the conclusion must be particular.
These rules are exceedingly important, as their observance is necessary in all mediate reasoning. The student needs, not only to understand the meaning of these rules, but he needs to commit them to memory so thoroughly that they may be recalled without hesitation or mistake. To aid the memory, the eight rules may be divided into these four groups:
I. Rules one and two relate to the composition of the syllogism.