My answer to Tobias’s question is as follows:
A human with ahetuka/dvihetuka (Bhikkhu Bodhi’s book has it as ahetuka/duhetuka) birth cannot attain magga phala OR jhanas in this human bhava.
– I think that is what Bhikkhu Bodhi is trying to say there. The discussion on that page is on (ahetuka) and double-rooted (dvihetuka) individuals.
– One MUST have a triple-rooted kamma (with all three “good roots” of alobha, adosa, amoha) giving rise to a “tihetuka birth” to be able to attain magga phala/jhana.
By the way, dvihetuka birth is brought by a kamma done with two “good roots”: amoha/alobha OR amoha/adosa.
– An ahetuka birth there DOES MEAN “without any roots” (i.e., no alobha, adosa, or amoha). That kamma may have just alobha or adosa without any understanding. The word “ahetuka” refers to the “helpless nature” of such a birth.
– While a dvihetuka person can accumulate punna kamma that can lead to a tihetuka birth in the future, a person who has an “ahetuka birth” is not capable of that. Those are persons born with severe disabilities.
Furthermore, it is NOT possible to determine whether a given person is dvihetuka or tihetuka.
– This is a tricky issue and sometimes people get discouraged whether they have a dvihetuka birth. For example, Ven. Culapanthaka almost gave up the robes because he could not memorize even a single verse. But he had a tihetuka birth. A previous akusala kamma had been “covering his mind”. Luckily the Buddha was there to see that and to give an appropriate kammatthana to overcome that situation. He attained Arahanthood overnight.
– So, we all should assume that we have tihetuka births and make effort. That effort will bear fruit either way.