2.58 When one is able to draw the senses away from the sense-objects on every side when the senses try to contact the sense-objects, just as a tortoise draws in its limbs, and is capable of fixing his mind on the self - he too is of firm wisdom. Thus there are four stages of devotion to knowledge, each stage being perfected through the succeeding stage.

Now Sri Krsna speaks of the difficulty of the attainment of firm devotion to knowledge and the means of that attainment.