THAT / அது

We name something only if it has opposite or alike. 
Example for opposite : Good – Bad, Day – Night 

Example for Alike : Human 

We will not name the sweet as sweet , unless we have tasted bitter or another sweet taste.
Incomparison with opposite or alike.
If bitter or another sweet taste is not there, sweet becomes nameless. 

Names gives individuality, in other words, separation from the whole.
What if, there is something, which is not having opposite or alike? 

If it is incomparable & how come the poor mind, which can only perceive from abstract, can know THAT?
So it is called as THAT/ ATHU/YEGAVASTHU naturally.