This was alleged erroneous; so please explain as though I were 10 years old. See the bolded in the following. 3 and 4 confuse me because they are premises, but they themselves are conditional sentences. So what do I do? How do I make sense of them?
5. If ALL of the following conditions are met:
1. We have not found any case where X present and Y absent.
2. We have tested a wide variety of cases,
including cases where X is present and cases where Y is absent.
3. If there are any other features [call hese F] that are always absent
where Y is absent,
then we have tested cases where those F are absent but X is present.
4. We have tested enough cases of various kinds that
are likely to include a case where X
is present and Y is absent, IF there is any such case.
6. [Then] we have good reason to believe X IS a sufficient condition of Y