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other meditative attainments

Imperturbable Attainmen

Ajaan Geoff mentioned [to friend] that the Vinaya also explicitly stated that the Imperturbable=mastery of the fourth jhana+any one of the formless bases. I forgot the reference, though (you can call and ask him).
In the Chinese Agamas, there's a sutra called the Budongdao jing, which also provides the same definition.
According to the Visuddhimagga, the Formless Bases require even more intense absorptions than the jhanas. And so if people can walk and talk while in the Formless Bases in Jhanas, would it not make sense that they can do so in the lower attainments which are the form jhanas?