To hope to obtain Brahman-realisation by just continuous thinking about it is like a man who wants to have a bath, starts all the way from digging up a well for the purpose.
But to reach the same goal through the mahAvakyas of the Upanishads is like drawing water from an already constructed well.
Of course you have to draw the water – not like opening a tap and using the downpour from it. The drawing of sufficient water from the well depends on the size of the bucket or the pail, the depth of the well and other factors.
The Samskaras of the individual influence the efforts to be made just as the smallness of the bucket will force you to draw water several times.
But when you compare this with the process of our digging up of a well – well, that is the comparison I mentioned.