Conventional wisdom holds that to see is to believe. So for us, what we can see with our own eyes is most convincing. For example, it would be quite difficult to visualize a transparent stone wall because the eyes do not see such a wall. However, when practice has reached a certain stage, practitioners will be much less influenced by the external factors. At that point, one has gained the ability to change or control outer phenomena at will, thereby weakening or eliminating the external influence altogether. But presently such ability is still beyond our reach. Although some may question its plausibility, descriptions of such ability are abundantly available in various texts and have been broadly analyzed in some of the more contemporary treatises. In addition, personal actualization by many accomplished practitioners has provided even stronger proof. It is just that our own practice is not up to that high standard yet. To get to that level of attainment, the foundational practices are absolutely indispensable. And the first step is to generate renunciation.

- Quote from The Right View, "The Four Noble Truths—the Path Out of Samsara"