|Chapter 2: Contents of the Gita Summarized|
avyakto 'yam acintyo 'yam
avikaryo 'yam ucyate
tasmad evam viditvainam
It is said that the soul is invisible, inconceivable and immutable. Knowing this, you should not grieve for the body.
The dimensions of the souls have been given to be as 1/10,000th of the tip of the hair, which for all practical purposes is invisible. Even this size is understood to be notional – to indicate that the soul is purely spiritual and has no material dimensions. Immutable means that the composition of the soul does not "mutate," or change. Finally the soul is called inconceivable to indicate that the soul cannot be conceived or understood by material or empiric knowledge. It can only be understood through the eyes of the scriptures.
Please read the Sanskrit Verse and the Prabhupada's Purport.