Atom is the ultimate and subtlest unit of matter. There cannot be any further subtler unit of matter other than an atom. Atom is indivisible.[1] In its existential aspect it remains alone. In that condition it is unitless. It cannot be parted or has no middle part of it and so it is partless.[2] Ṭhāṇaṃ explains atom as unpierceable, incombustible, inconceivable, having no middle part, having no half, unit-less and indivisible.[3] In Bhagavatī, there is mention of four kinds of atoms substance-atom, space-atom, time-atom and mode-atom. Here substance atom is explained as impossible to cut, burn and catch; space-atom is explained as having no middle part, no half, no unit and indivisible.[4] The nature of substance-atom and space-atom as explained in Bhagavatī, is similar to the nature of atom that has been given in Ṭhāṇaṃ. The only difference is that in Ṭhāṇaṃ, it has been explained in combined form without mentioning about substance and space distinctively. In Bhagavatī, at some other place, atom has been defined as that which is devoid of half part, having no middle and unit-less.[5]
Atom is unitless- this statement is from the view point of substance and space. From the point of view of time and modes, it can be both unitless or with units.[6] From the view point of substance the atom has no parts so it is unitless and from the view point of space, the atom resides in only one unit of space so it is unitless. From the viewpoint of time, the atom which is stable for one instant of time is unitless. This means that after one instant of time it would change its form and become an aggregate and the atom which will remain as atom for more than one instant, will be atom with units. Atom has the qualities of colour, taste, smell and touch in different variations. An atom can be possessed with infinite points of black colour whereas another atom may have only one degree of black colour. Same is the variation found regarding other qualities like smell etc. also. So when an atom is possessed of one degree of colour, taste etc. then it is unitless and when the degrees are more than one, then atoms have units and are called sapradeśī.[7]
Bhagavatī states that mode-atoms are possessed of colour, smell, taste and touch.[8] Due to this, they can be called as sapradeśī (with units). Siddhasena gani has also called mode atom as 'with unit' and substance- atom as 'without unit.[9]' Through anekānt, this concept of 'atoms having unit' and 'unitless atoms' can be established.
Aggregate - with Unit or Unitless
Just as an atom, an aggregate can also be both 'with unit' and 'unitless'. This can be proved from a relative view point. As we have discussed above from the view point of substance, an atom is 'unitless'. It is the opposite in the case of an aggregate. From the view point of substance, the aggregate would always be 'with units.[10]' From the view point of space, time and modes, it is both 'unitless' and ' with units'. From the view point of substance the aggregate having two units etc. are always sapradeśī (with units). From the view point of space, the aggregate residing on one unit of space is 'unitless' and which is accommodated on more than one space units is with unit. From the view point of time, the aggregate of one instant of time is ' unitless' and that which remain stable for more than one instant of time is 'with unit'. From the view point of modes, the aggregate having one degree of colour etc. is ' unitless' and that which has more than one degree of colour etc. are with unit.[11]
Eternality and Non-eternality of Atom
According to Jain philosophy, every existent is a composition of mutually contradictory attributes. These contradictory attributes coexist in the substance simultaneously and at same time. This is also the case with atoms. They are both eternal and non-eternal.[12] From a substantial view point, it is eternal and from the point of view of its modes like colour, smell, taste and touch it is non-eternal.[13] From a relative perspective, we can well describe the co-existence of contradictory attributes.
Prajñāpanā Vṛtti, p. 202-3.
paramaṇurhi apradeśo gīyate dravyarūpatayā sānśo na bhavatīti... kālabhāvābhyām sapradeśatvepi na kaściddoṣah.
... nanu prasiddhamevedameka rasagandha varṇo dviśaparśaścāṇurbhavati,
bhāvāvayavaiḥ sāvayavo dravyāvayavairniravaya iti.