What do we meditate on? How do we develop insight? This is a very important question.
There are two kinds of meditation: meditating to develop calm and meditating to develop insight. Meditating on the ten devices only gives rises to calm, not insight. Meditating on the ten foul things (a swollen corpse, for example), too, only gives rise to calm, not insight. The ten recollections, like remembering the Buddha, the Law and others, too, can develop calm and not insight. Meditating on the thirty two parts of the body, like hair, nails, teeth, skin - these too, are not insight. They develop only concentration.
Mindfulness as to respiration is also concentration developing, but one can develop insight from it. Visuddhi Magga, however, includes it in the concentration subjects and so we will call it as such here.
Then there are the four divine states, love, pity, sympathetic joy, and equanimity, and four formless states leading to formless jhanas. Then, there is the meditation on the loathsomeness of food. All these are subjects for concentration meditation.
When you meditate on the four elements inside your body, it is called the analysis of the four elements. Although this is a concentration meditation, it helps develop insight as well.
All these forty subjects of meditation are subjects for developing concentration. Only respiration and analysis of elements have to do with insight. The others will not give rise to insight. If you want insight, you will have to work further.
To come back to our question, how do we develop insight? The answer is: we develop insight by meditating on the five aggregates of grasping. The mental and material qualities inside beings are aggregates of grasping. They may be grasped with delight by craving in which case it is called "grasping of the sense objects" - or they may be grasped wrongly by wrong views - in which case it is called "grasping through wrong views." You have to meditate on them and see them as they really are. If you dont, you will grasp them with craving and wrong views. Once you see them as they are, you no longer grasp them. In this way you develop insight. We will discuss the five aggregates of grasping in detail.