«وسالبة: وهي التي حكم فيها بنفي شيء عن شيء»
And the second is negative, in which something is denied of something else.
«والإنسان ليس بفرس»
And the human is not a horse.
Here “human” is the subject and “horse” the predicate, but the predication is denied rather than affirmed. “Negative” does not mean meaningless or false: “A human is not a horse” can be a true negative proposition.
Affirmative and negative are distinguished by the affirmation or denial of the predicative connection, not by approval or disapproval.