It is not just reflecting on karma and its consequences that we practice in this way; we contemplate like this for each of the four thoughts. For example, with the precious human life, as was explained before, we have the eighteen different perspectives from which to reflect on the preciousness of human life. We should do the same for each of the preliminaries, contemplating and reflecting after we obtain an understanding of what we are going to be thinking about. Not all meditation involves contemplation; there are obviously very many different types of meditation, some of which involve contemplation, and some involving visualisation. But for the purposes of the preliminary practices it is essential that we contemplate on these four thoughts, because what we are seeking to do is overturn our hitherto mistaken values and ideas. In order to do this we must think, contemplate, and reflect; simply meditating on nothing would not achieve this result.
Depicted from "Guide to the Four Preliminary Practices 3 - Part 35" April, 2015, Vancouver, Canada