Concentration
Moreover, although there are countless things which may appear as that
on which one can concentrate, in brief, there are four:
1. The objective reference which purifies the conduct of life
2. The objective reference which purifies emotions
3. The objective reference which are the (four) encompassing
activites (of loving kindness, compassion, joy and equanimity)
4. The objective reference with which the wise deal
The objective reference with which the wise deal is said to be limitless
and refers to such topics as the five constitutive elements, the three
levels of existence, the six perceptual spheres, dependent origination
and so on.
Nowadays, some people who take the Buddha's words out of context and make a display of meditation, meditate by concentrating on what appears before their visual perception, but Asanga has stated very clearly that concentration does not take place in sensory perception but in categorical perception, and the objective reference is not the visible object that is present in sensory perception, but its precept.
Now if you think that the object of intense concentration must necessarily
be a real object, this is not the case. You can concentrate on anything
-- be it a real or fictitious object. If you work on it intuitively, you
get from it a pure and non-conceptualized vision.
Whether it is real or not,
Whatever becomes truly familiar
When you have become fully conversant with it
Results in a clear feeling of presence without subject-obejct dichotomy.
Through a widened perspective which is together with calm,
One must first become calm, and this comes
When one is not attached to the world