Freawaru wrote:Yes, this seems to be a part of my confusion. For in practice they don't seem to be as easily discernable as in theory.... If I understand you correctly, in the Theravada system this observation of the five aggregates is vipassana and in the Tibetan system samatha. But, frankly, I recognized aspects of both - such as stability of concentration on an (nimitta?) object and increase of discernment and recognition.
As Dave already alluded to here
, samatha and vipassanā in the Suttapiṭaka and Abhidhammapiṭaka refer to two qualities of mind, and bhāvanā refers to the development or cultivation of these two qualities of mind. It's only later, in the classical period of commentary, with the large systematic texts such as the Vimuttimagga, the Visuddhimagga, the Abhidharmakośa, and the Śrāvakabhūmi (which are all great systematic treatises branching off of the Indian Sthaviravāda), that we see the somewhat arbitrary separation of samathabhāvanā and vipassanābhāvanā according to different subjects of meditation.
Freawaru wrote:Is it possible to develop stability of concentration on one object (samatha) simultaniously with momentary concentration that observes the five aggregates? So that there are two different kinds of concentration simultaneously there? Like two hands?
Well, worded differently, yes. See, for example, Contemplation of the Mind: Practicing Cittānupassanā
by Ven. Khemavamsa (esp. page 8 and then touched upon throughout the text).
What you are referring to was developed within Indian Sautrāntika and Yogācāra Pramāṇavāda (Epistemology). It was also developed in somewhat different terms in the Kagyu treatises on co-emergent mahāmudrā.
All the best,