Examined sentence: "On this island is a treasure, only if I am an honest man."
1st part - On this island is a treasure
2nd part - I am an honest man
Swindlecant:
1st part - ?
2nd part - false
So since the whole sentence has to be lie, the 1st part has to be true (see truth table above). So there is a treasure.
Honestant:
1st part - ?
2nd part - true
So since the whole sentence has to be true, the 1st part has to be true (see truth table above). So there is a treasure.