Introductory Remarks -- What this section appears to me to be about:

Venn Diagram (human(slave))

being a slave → being a human.
S → H

S H S → H
1 1 1
0 1 1
1 0 0
0 0 1

Paradigmatic case: S = 1  H = 1
Contrary case:        S = 1 H = 0

"是而然" indicates situations under which the antecedents and consequents are true statements about the world and the implication is true.

The Text:


A white horse is a horse. To ride a white horse is to ride a horse.
A pure black horse is a horse. To ride a pure black horse is to ride a horse.
A prisoner of war is a human being. To love a prisoner of war is to love a human being.
A slave is a human being. To love a slave is to love a human being.

    • Commentary: If the above propositions are restated in the form S → H, then both S and H would be true in the paradigmatic instance, and S would be true but H would be false if a counter-case were encountered (e.g., a slave that happens to be a Martian). So for the implication postulated to give a false result we would have to find a something like a Martian slave.

    • What we can note about all of these propositions is that the color or condition of servitude is irrelevant to how they are treated, i.e., the sub-sets are not very relevant to the verbs. While we might care what color of horse we are riding, as far as riding horses in general goes, any color of horse will do. So if we say: "I am riding a white horse," the relevant thing is that I am riding a horse; I think Aristotle would regard its whiteness as an "accident." Ordinary people look at situations described  in the Chinese sentences and believe that they are valid statements. In other words, ordinary people find these ideas reasonable.