«وشرط لتحقق التناقض بين القضيتين المخصوصتين وحدات ثمانية»
Eight kinds of identity are conditions for establishing contradiction between two determinate propositions.
They are:
1. Identity of subject
2. Identity of predicate
3. Identity of place
4. Identity of time
5. Identity of potentiality and actuality
6. Identity of condition
7. Identity of part and whole
8. Identity of relation
The list is not merely terminological. Every condition tests whether the statements actually affirm and deny the very same thing.