Calm (samatha)
& Insight (vipassana):

There are two principal kinds of mental development:
1: Development of mental
tranquillity and
calm
(samatha-bhāvanā)
culminating in the development of
concentration
(samādhi-bhāvanā)
And!
2: Development of
insight
(vipassanā-bhāvanā)
culminating in the
development of
understanding
(paññā-bhāvanā).
Both qualities:
Calm and
Insight, are essential & crucial for release!
Tranquil Calm
(samatha) is the pleasant, peaceful, yet lucid state of
a settled mind acquired as a preliminary to meditative absorption.
It blesses the meditator with 3 things: Happy life, happy rebirth, &
a mental purity suitable for gaining progressive penetrating insight!
Concentration is a necessary prerequisite for such cutting insight..
Why so? It removes distractions that veil the investigating vision!
Insight is that which leads to entrance of the
4 stages of Nobility..
Why so? It irreversibly removes gross &
latent
mental hindrances!
The term
samādhi literally means ~being firmly put evenly together
(sam + a+ dha) & is mental state focused on only one single object=
cittassaekaggatā,
which literally means ~ one-pointedness of mind.
Any state of consciousness has a degree of mental concentration!
One may distinguish these four stages or levels of concentration:
1: Momentary or transient
concentration
(khanika-samādhi),
2: Preliminary or preparing
concentration
(parikamma-samādhi),
3: Access or approaching
concentration
(upacāra-samādhi),
4: Absorption or attainment
concentration
(appanā-samādhi).
Insight
(vipassanā)
is the penetrative
understanding, gained by only
direct meditative experience of the inherent
transience,
misery, &
selflessness
(anicca,
dukkha, anattā) of all physical & mental states
of existence all included in these
5 clusters
(khandha)
of clinging:
form, feeling, perception, mental construction, and consciousness.
Bhāvanā
is derived from the causative form of the verbal root
bhu,
bhavati
=
to be, to become, causing to be, making come into being,
the calling into existence of, the production of or development of.
When applied to Mental Training this simply means
Meditation...

Simultaneous Calm and Insight!
Photo: The Kawasakis
More
on Insight (Vipassanā):
http://What-Buddha-Said.net/drops/II/Highway_to_Nibbana.htm
http://What-Buddha-Said.net/drops/II/The_18_Principal_Insights.htm
http://What-Buddha-Said.net/drops/IV/Breathing_Calm_and_Insight.htm

More on Calm (Samatha):
Forest
Bliss, Calm, Calm_and_Insight, Calm_Power, The_Tranquil_One,
Breathing_Calm_and_Insight, Silenced, Forest_Bliss2, Calmed...
:-)