ならいざ知らず

ならいざ知らず is used to contrast two things, emphasizing that what follows is less likely, less appropriate, or more difficult than what was just mentioned. The thing before ならいざ知らず is treated as a special case or exception; the speaker acknowledges that the first part is acceptable, possible, or understandable in its own right, but not in comparison with the second part. It often introduces a more difficult, unrealistic, or inappropriate scenario to be contrasted with a more favorable or typical one. Typical contexts involve shifting from a relatively easy or obvious case to a much harder or less appropriate one.