"Atman" or self or soul is imperishable. "Jagat" or the world is perishable. This discriminating ability of the mind or intellect is known as "Viveka." Viveka is the root means, because without this discriminating ability, the subsequent means can not be developed and mastered.
If you are new to Vedanta, please refer to some of the discourses in the Jnana Yoga (Yoga of Knowledge) section of this website. Thank you.