Examining samatha and vipassanā by comparison from several perspectives shows the following:
First, in both samatha or vipassanā, it is imperative to establish the meditation object clearly. Seeing from the mundane state, the object of samatha is a sign [nimitta], a concept [paññatti]. The object of vipassanā is phenomena [dhamma]. This is the most important measuring stick to distinguish samatha from vipassanā.
Second, samatha is a practice of developing concentration by focusing on an object—a sign. Vipassanā is a practice of seeing, with insight, the impermanence·suffering·non-self of the conditioned phenomena. Samatha is the serene state of mind achieved by concentrating the mind on a sign and steadying tremors and disturbances of the mind. Therefore, in Chinese, it is translated as 止 [concentration]. With vipassanā, one does not see the object from a conventional mode but sees the impermanence·suffering·non-self, the characteristics of dhamma, with insight, in accordance with vipassanā’s literal meaning of ‘deconstruct [vi] and see [passanā].’ Therefore, in Chinese, it is translated as 觀 [seeing].
Third, the key word for samatha is a sign and the key words for vipassanā are the impermanence·suffering·non-self. In the Path of Purification, there are 40 meditation objects of concentration. If one takes a narrow definition of the samatha practice as a practice to obtain absorption-concentration (M.A.ii.346), samatha is about taking one object among the 22 meditation objects of the 40 meditation objects, then one concentrates the mind on it and creates a learning sign [uggaha-nimitta] from the object. Eventually this learning sign is purified into a counterpart sign [patibhāga-nimitta] that does not scatter and becomes whole. At this point, the mind is extremely focused on this counterpart sign each and every moment. Vipassanā is a practice of seeing with insight one of the 71 ultimate realities that are further classified as mind, mental factors, and materiality. In this way, when one sees the dhamma with insight, the impermanence·suffering·non-self can be clearly seen.
Fourth, there are different kinds of concentration: the concentration attained through samatha is access-concentration [upacāra-samādhi] or absorption-concentration [appanā-samādhi]; intense concentration attained during vipassanā practice is momentary-concentration [khanika-samādhi].
Fifth, deliverance·Nibbāna cannot be realized through the serenity of samatha only. Because samatha is a state in which the mind and the meditation object become one, the lust·hatred·delusion are suppressed and lie dormant by clear and bright serenity. However, once one is out of samatha, the lust·hatred·delusion’s influence returns. This state is called ‘temporarily liberated’ [samaya-vimutta] in the discourses. (A5:149) Therefore, the lust·hatred·delusion must be eradicated through the power of vipassanā that clearly sees the impermanence·suffering·non-self with insight. As a result, those taints will not arise again and deliverance·Nibbāna can be realized.
Without the wisdom of vipassanā, deliverance is impossible. However, it is definitely not easy to obtain the wisdom of vipassanā without the aid of samatha. Therefore, in the Early Buddhist discourses, the technical terms like samatha and vipassanā almost always appear together. The Buddha emphasized practicing these two diligently.
Sixth, whether one should practice samatha first, vipassanā first, or practice both simultaneously is ultimately up to one’s teacher or one’s own interest and preference. In the In Conjunction Discourse, the Buddha said all states from first jhāna to ‘cessation of perception and feeling’ become firm bases for the attainment of Arahantship. (A4:170) Of course, one who practices dry vipassanā [sukkah-vipassaka], in other words, pure vipassanā [suddha-vipassaka] can only obtain the Arahantship by practicing the dry vipassanā without the moisture of jhāna. (DA.i.4)
In conclusion, one may practice samatha first, vipassanā first, or samatha and vipassanā simultaneously. Saying that one must practice samatha first or vipassanā first is a mere dogmatic opinion; one cannot be called a true practitioner while insisting so. In any circumstances, the important matter is that the Buddhist practice always ends up in vipassanā, insight of the impermanence·suffering·non-self. Vipassanā mentioned here is seeing clearly the impermanence·suffering·non-self and does not refer to any special specific practice method.
“Samatha is a practice in which one focuses on an object called a sign and develops concentration. Vipassanā is a practice in which one sees clearly the impermanence·suffering·non-self of dhamma and develops insight.”
[Introduction to Early Buddhism authored by Bhikkhu Kakmuk and translated by Nancy Acord]
 Nimittas refer to signs or images.
 The ‘moisture of jhāna’ means the rapture and happiness of jhāna.
 The gotrabhū is one who has entered the lineage of the Noble Ones.