The sole of your hiking boots should have a rough pattern, which gives you a good grip on both loose and wet surfaces. Let the terrain determine the thickness of the sole. When walking on rocks or in other sharp terrain, you should have a thick enough sole so that you never have to feel the surface through the sole. If you walk a lot in uneven terrain, it can be an advantage to have a lower sole, so that you get closer to the ground and reduce the risk of treading wrongly.