due to forgetting carpentry? Yes, he is liable for it, because the two trades are now different in kind: carpentry and blacksmithing.

His statement, “But if it is of the same kind, he is liable only for whichever increase is greater,” applies, for example, if he learns computer skills in one particular field, then learns them in another field and forgets the first skill. In this case, the increase is of the same kind as that which he forgot, so he is liable only for whichever of the two increases is greater. Thus, if the increase came after the decrease, he bears no liability, because he has made him better off. But if what he forgot was greater, then he is liable for the greater amount, because the two are of one kind.