Concentrations are of different kinds, and their difference is caused by the difference of the purpose.
There is a concentration on an object or a person or on an affair, which results in knowing all about the object of concentration. This is the concentration of a student, who receives knowledge by concentration. It is such concentration as looking at a flower and thinking of it with closed eyes, and thinking what flower it is, what fragrance it has, what colour it has, why it has this fragrance, what is its significance, what nature it has, what is its secret.
Another concentration is the concentration of a psychic, who concentrates upon a certain object in order to exercise his mind, and when the mind develops power then he utilises it in all things he wishes to accomplish in life.
There is a concentration of the idealist, who admires a hero of the battlefield or a king in his grandeur or a leader, a teacher, a prophet, and by so idealising, he acquires in himself by the way of concentration the qualities of the idealised hero. By a close study of history you will verify this in the life of great people, for most of them became great by idealising, admiring a great person and thinking of a great person.
There is the concentration of the lover. It is still stronger, because the lover, in his devotion to his beloved, naturally forgets himself, and the secret of all spiritual progress lies in this one thing, the forgetting of self, which a lover, a devotee accomplishes without any special effort, because he cannot help but think of the object he loves. He need not hold his mind on a certain object by force of will, on the contrary his difficulty is to get away from the thought. A wandering mind is natural in the average person, and that is different from the lover. Therefore the difficulty of which the average person complains, of keeping of the mind steadily on one subject, is the simplest thing to the devotee. It is therefore that Sufis have recognised devotion as the best means of spiritual attainment, and many among them walk in the path of love.
Concentration upon a rock must naturally give the rock quality, the heart must in time become like a rock. Naturally concentration on a flower should produce beauty in mind and body. So concentration on the brave gives bravery, on the great gives greatness, and concentration on the holy ones gives holiness. This proves that no common object can be recommended as the best for everyone to concentrate upon, as one medicine cannot be a prescription suitable for everybody. The object of concentration must be chosen according to the purpose one wishes to accomplish in life.