 Schmithausen notes that the mention of the four noble truths as constituting the core of "liberating insight", which is attained after mastering the Rupa Jhanas, is a later addition to texts such as Majjhima Nikaya 36.Schmithausen discerns three possible roads to liberation as described in the suttas, to which Vetter adds the sole practice of "dhyana" itself, which he sees as the original "liberating practice"