Anu is a unit of length in Hindu science, defined as the fourteenth part of an angula (roughly one inch), as specified, for example, in the section of Baudhayana Srauta Sutra dealing with the building of the altar for the ritual. Anu is little less than 2.5 times the size of a tila (sesame seed).
In Hindu philosophy and science, anu is the ultimate
material unit in the physical world, corresponding to the atom of Greek
philosophy, and (for this reason) is often translated into English as ‘atom’.
There are, however, important differences between the Greek notions of the atom
and anu or paramanu.
In the Samkhya Philosophy, there are primordial subtle
essences of the five elements which pervade the universe. These are called
tanmatras. Tanmatras contain the five roots of our sensations – Shabda (which
can be heard), Sparsha (which can be touched), roopa (which can be seen), rasa
(which can be tasted) and Gandha (which can be smelled).
In Vaiseshika, the combination of two anus produces a
dvayanuka (or dyad) which is without extension. Three dvayanukas together form
a trayanuka (or triad) which is the smallest visible unit of matter. Further,
dyadic combinations are also possible. Combinations of anu (s) arise from
invisible factors, referred to as adrishta. Adrishta is a metaphysical notion
that extends also to souls.
Anu -s and paramanu -s are eternal, and are of different
kinds. The mahabhuta -s (the basic elements of ancient science) are prithvi (earth),
ap (water), tejas or agni (fire), vayu (air) and akasha (the ubiquitous one).
Of these, the first four are said to be made up of paramanu-s. Combinations of
paramanu-s result in the formation of the gross body (sthulabhuta), which
constitute the tangible, perceivable things of the physical world.
The paramanu -s of different substances have different qualities.
Thus, the paramanu-s of Prithvi (earth) are endowed with the qualities of
color, taste, odor, touch and sound; those of ap (water) with color, taste, touch
and sound; those of tejas (fire) with color, touch and sound; those of vayu
(air) have only touch and sound; and akasha only sound.
In addition to these, various paramanu-s have a number of
other qualities of which number, dimension, distinctiveness, conjunction and
disjunction are common to all.
A mote in a sunbeam is called an anurenu, and its size is
estimated to be the order of 1/34925 of an inch. A related concept is of truti,
an infinitesimally small unit of time, a 54,675,000th part of a muhurta (about
48 mintues). Thus, truthi is of the order of 5 milliseconds.
Several thinkers, such as Prasastapada, Udyotakara, Vacaspati Mishra, Sankara Mishra, Sridhara, elaborated on the Vaiseshika views of anu. On the other hand, Shankara, in his Raghunatha Siromani commentary on Brahmasutra, argues against the concept of anu.