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Source: *A Concise Introduction to Logic* (12 Ed, 2014), by Patrick J. Hurley

[p 286:] Rule 1: The Middle Term Must Be Distributed at Least Once.

[p 288:] Rule 4: A Negative Premise Requires a Negative Conclusion,

and a Negative Conclusion Requires a Negative Premise.[p 288:] Rule 5: If Both Premises Are Universal, the Conclusion Cannot Be Particular.

[p 290:] [...] The question remains, however, whether a syllogism’s breaking none of the rules is a

sufficientcondition for validity. In other words, does the fact that a syllogism breaks none of the rules guarantee its validity? The answer to this question is “yes,” but unfortunately there appears to be no quick method for proving this fact. [...] The proof that follows is somewhat tedious, and it proceeds by considering four classes of syllogisms havingA,E,I, andOpropositions for their conclusions. [...][p 291:] Next, consider a syllogism having an

Iproposition for its conclusion.

By Rule 1, M is distributed in at least one premise,

and by Rule 4, both premises are affirmative.

Further, by Rule 5, both premises cannot be universal. [...]

I do not comprehend only the step bolded above:

How does Rule 4 imply that both premises must be affirmative?