 Assume that ‘Al did not run’ negates ‘Al ran’, while ‘Bob did not swim’ negates ‘Bob swam’. Then (9) is an instance of the following valid form: if A then either not-A or not-B, and A; so not-B. But we can treat this as a derived form, by showing that any instance of this form is valid given two (intuitively more basic) Stoic inference forms: if the first then the second, and the first, so the second; either not the first or not the second, and the first; so not the second. For suppose we are given the following premises: A; and if A, then either not-A or not-B. We can safely infer that either not-A or not-B; and since we were given A, we can safely infer not-B. Similarly, the syllogistic schema (10) can be treated as a derived form.