Concentration should be followed by meditation.
Meditation is nothing but protracted or sustained concentration.
A scientist has to concentrate on a problem, on a given subject, on a riddle, to bring out the answer, to solve it.
He has to think, think and think.
Then only the answer flashes forth.
Likewise, meditation is intense concentration, concerted concentration on the problem of life, on the problem of the inexplicable triad of God, man and the universe.
While concentration becomes essential even to solve small problems in science, what to speak of the problem of life which has baffled humanity since time immemorial?
The Sadhak (aspirant) who wants God must meditate, meditate and meditate.
Meditation can be practised on any image of the Lord.
This is concrete meditation.
After some practice, the aspirant will be able to visualise the form of the image even with closed eyes.
Meditation can also be practised on abstract ideas and on various Vedantic formulae such as "I am Eternity", "I am Infinity" and so on.
To be continued ...