In Objective Concentration, Pratyahara and Dharana are the preparatory stages. Take a scientist, for instance. He knows that when the mind is engaged with several things, mind force is scattered. He cannot be a politician, a musician, etc., and at the same time an expert scientist. He gradually abstracts his attention from all other subjects and pauses it on one subject or one set of subjects.
Pratyahara is the continued effort of the mind to so abstract itself.
Dharana is reached when this effort is finally successful and the mind becomes steadfast and one-pointed. Dhyana is an extension of this steadfastness. When Dhyana is reached, the student is beyond the range of books. His mind is occupied with original researches and experiments and his knowledge becomes more and more definite. Going on and on always on the one line complete knowledge of that subject is attained. This is the objective view of Samadhi. All these stages when completed make one Samayana. The subjective view of Samadhi no books or writings can teach you. As you go deeper and deeper into Yoga, you will understand these things in the light of your Soul-Vision. It will come to you if you follow my subsequent instructions.