The first 3 dimensions are easy for people to understand but then we need some assistance. To imagine the 4th dimension, we think of walking in a forest along a footpath. That could be thought of as being our experience with the footpath being the 4th dimension of duration. We walk along the footpath as we age from the past to the future. We cannot back-track along the path. As we walk along we see there are other paths along the footpath which branch off and we have to make decisions about which route to follow. Once we have passed a junction we cannot go back and choose another path. This is our reality.