«التناقض هو اختلاف القضيتين بالإيجاب والسلب بحيث يقتضي لذاته صدق إحداهما كذب الأخرى أو بالعكس»
Contradiction is a difference between two propositions in affirmation and negation such that, in themselves, the truth of one entails the falsehood of the other, and conversely.
Affirmation and negation must concern the very same predication. “In themselves” means that the exclusion follows from their logical relationship, not merely from accidental external circumstances.
The textbook's example is:
«زيد قائم، وزيد ليس بقائم»
Zayd is standing; Zayd is not standing.
With the same subject, predicate and relevant circumstances, the two cannot both be true or both be false.