The book contrasts Zayd's state at night with his state by day:
“Zayd sleeps at night.”
“Zayd does not sleep in the daytime.”
Both can be true because they concern different times. A person may sleep at night and remain awake by day.
A genuine pair keeps the time fixed:
“Zayd is sleeping now.”
“Zayd is not sleeping now.”
When time qualifies the judgment, changing that time changes the proposition. That is why temporal identity matters.
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Contradiction: Definition, Conditions and Application to Propositions
The book contrasts Zayd's state at night with his state by day:
“Zayd sleeps at night.”
“Zayd does not sleep in the daytime.”
Both can be true because they concern different times. A person may sleep at night and remain awake by day.
A genuine pair keeps the time fixed:
“Zayd is sleeping now.”
“Zayd is not sleeping now.”
When time qualifies the judgment, changing that time changes the proposition. That is why temporal identity matters.