If meditation means a thoughtless state of mind then a stone should be enlightened?

Good question! A dead body is also thoughtless and therefore enlightened? Eventually everybody dies and everybody is enlightened as soon as they are dead :). Difference between a living and dead is the life force energy i.e prana . There are different units of prana. A stone has one unit of prana, water has 2, fire has 3, air has 4, Plants have 5 and animals have 6 units of prana. Human beings have the capacity to have anywhere between 7 and 16 units of prana. When we are depressed and low in energy, our prana level is less ( like 7) below that we behave as animals:) and way below that we become suicidal and completely stoned ( sic). :) A fully blossomed enlightened being is said to have the highest unit of prana- 16. So there is a humongous difference between being lifeless and thoughtless and bubbling with life and being thoughtless :).