1. The original: “Some plants are not-trees.”
2. Negating predicate: “Some plants are trees.”
3. Changing to an O: “Some plants are not trees.”
Dropping the not from “1” and then adding it again to “2” is simply putting into operation the double negative idea, so that there is no violation of the principle.
The O Proposition.
O bears the same relation to I that E bears to A. The principle involved is the same. The process is illustrated by reading reversely the scheme of illustrations under I. The rule is as follows: To obvert an O negate the predicate and change the O to an I by eliminating the not.
Summary of Obverting the Four Logical Propositions.
1. Principle:
Two negatives are equivalent to one affirmative.
2. Rules: