Chapter 2,Verse 25

Avyakto'yam achintyoyam
avikaryo'yam uchyathe
tasmad evam viditvainam
nanusochithum arhasi


Naturally,the next question would be-"If atma is everywhere,they why cannot I sense it or perceive it.I have sense organs which enable me to touch,smell,see,taste and hear.Why is that the atma is not sensed by any of these?"Krishna expects this question and hence gives an answer to Arjuna.He says that atma is "Avyaktaha",meaning it does not have sensory property.My sense organs can experience something that has one of the five sensory properties.If an object has a form and/or colour,I can see it.But when it has neither,can I see it?
Can I see the ethyl mercaptane,very strong smelling substance which is mixed with LPG to detect gas leaks?Just because I cannot see it,can I assume that there is no gas leakage?Just because a deaf person cannot hear music,can he conclude that there is no such thing as music?These only mean that out sense organs are limited instruments which can perceive what they are meant to perceive.Similarly,atma,having no sensory property,cannot be perceived by any of our sense organs.They are intrinsically incapable of perceiving the atma.Thus atma is imperceptible.

Krishna goes one step further and answers another question-"I cannot sense the atma.But why cannot I intellectually think or amotionally conceive the atma?"
The answer is "Achintyoyam",meaning,non availablity to form a concept.All the concepts we conceived are based on our experiences in this material world.Thus,we cannot conceive something we have not experienced.Dont agree?Try explaining colours to a person born blind.Let's see you make him perceive/conceptualize and imagine them.Thus,atma cannot be imagined either.

"avikaryo'yam uchyathe" means that the atma is not subject to any change,as I have explained in the previous posts.

Krishna says that,having known this,that Arjuna is neither a kartha nor a boktha,he cannot commit papa or punya and incur their consequences,he must not grieve over the death of his opponents.He is the punya-papa-atheetha atma.