Anuruddha (2002, Summary of the Topics of Abhidhamma and Exposition of the Topics of Abhidhamma, R. P Wijeratne and Rupert Gethin (translators) writes:
The earth element, water element, fire element, and wind element are called 'materiality of the elements'.
Earth (pathavi) is in the sense of extending (pathana). It is said that, like the earth, the natural state (pakati) of trees and mountains, etc., is clear (pakkhayati), stands out (upatthati), by their being the basis (patitthana) for other conascent materialities.
The earth element is the earth which is itself an element because it bears, etc., its own characteristic mark in the sense that it is insubstantial and lifeless, and because it is similar to those constituent elements of the body that are hard.
Water (apo) is that which causes conascent materialities to pervade (apeti), it spreads them; or it is that which enlarges (appayati) them, it causes them to grow and strengthen.
Fire (tejo) is that which fires (tejetti), that which cooks or sharpens; by its sharpness it heats the other three elements.
Wind (vayo) is that which blows (vayati); by being the cause of their arising in a different place, it moves a collection of elements. However, it should be understood that these four have respectively the characteristics of hardness, fluidity, heat, and distending.
Materiality (rupa) is the state of affliction (ruppana); materiality is also that which is associated with this [state], as in the cases of 'suffering from piles' (arisasa) and 'a blue lotus blosssom' (niluppala). As the word 'form/materiality' (rupa) commonly occurs other than in its technical sense, so as in the expression 'suffering as pain' (dukkhadukkha), form as materiality (ruparupa) is stated to distinguish it from this other [usage of the] word rupa.
Materiality for observation is such because, by being distinguished by a particular nature, without reference to the state of delimitation [by space], etc., it is suitable for observation by focusing on the three characteristics.
Four elements, five sensitive materialities, four sense-objects, the twofold sexual condition, materiality of the heart, along with the pair life and food make eighteen; these and delimitation, twofold communication, threefold alteration, fourfold characteristic (which, since they are not produced by specific causal conditions separate from the state of delimitation and alteration, etc., of materialities, are ten that are not real products (anipphanna) constitute twenty-eight kinds of materiality.
Eight kinds consisting of colour, taste, smell, nutriment, and the four elements are inseparable materiality; the rest are separable materiality.
Colour (vanna) is that which should be pictured (vannitabba), should be seen.
Nutriment (oja) is that which generates (janeti) materiality immediately upon its arising (udaya).
These are inseparable materiality since they do not occur individually separated from each other in any circumstances. The ideas of those who argue that in form world smell, etc., do not exist have been refuted by teachers in various places.
Thus in this way those with insight analyse the twenty-eight different types appropriately as internal, etc.
In summary, it should be noted that
(1) all the 28 rupas arise in the kama-sphere;
(2) in the 15 rupa-planes, with the exception of asaņņa-satta, 23 rupas arise (ghana-pasada, jivha-pasada, kaya-pasada, itthi-bhava and purisa-bhava being excluded);
(3) only 17 rupas arise in the asaņņa-satta plane; they are 8 avinibbhogas, jivita, pariccheda, 3 lahutadi, 4 lakkhana rupas;
(4) no material arises in the arupa-planes;
(5) eight rupas comprising sadda, five vikara-rupas, jarata-rupa and aniccata-rupa, do not arise at the moment of birth (patisandhi-kala) whereas, during life-time, there is no rupa which is not formed (Mon 1995: p. 250).
References
Anuruddha, 2002, Summary of the Topics of Abhidhamma and Exposition of the Topics of Abhidhamma, R. P Wijeratne and Rupert Gethin (translators), Pali Text Society, Oxford, pp. 217-218, 223-224, 229-230, 235
Dr. Mehm Tin Mon, 1995, "The Essence of Buddha Abhidhamma", publisher Mehm Tay Zar Mon, Yangon, p. 250
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