I believe that before we can start talking about existence, we have to first study science. Our world is defined by physical laws, but there is no reason for these laws to be the way they are. The speed of light is a constant, gravity pulls objects together, no one knows why or how these things came to be.
One reason which may be suggested is that we exist to observe these phenomenon precisely because we are the product of it. Thus, it is unsurprising that the physical laws of our universe fit our needs so closely. In a way, the universe is self-selecting in that we cannot exist to observe a world where gravity drives us apart. This is known as the anthropic principle. However, this would also render the study of existence pointless, as we can only observe a universe with us in it. We cannot hope to understand what goes on beyond our universe, even though the origin of our existence likely lies beyond what is present here.
Another point of interest is the direction of time. For 3-dimensional beings such as ourselves, time is linear and always flows forward. We age, society changes and adapts, all of this happen in the forward direction of time. But we have to ask ourselves, why does time necessarily have to flow forward?
Now, there are two arrows which set the direction of time. The psychological arrow that we all know, which is what we perceive as the forward motion of time, and the thermodynamic one, which is the direction in which entropy increases. Entropy is the measure of the disorderliness of things. When we drop a cube of sugar into tea and it dissolves, we say that the entropy of the sugar and tea has now increased as the particles of sugar are more randomly distributed within the particles of tea.
Chances are you have seen a vase fall onto the ground and shatter, but you have never see the pieces of glass pick themselves off the floor and reform into the vase on the table. Why is that so? That is because in any self-contained system, entropy can only increase with time, never decrease. A similar concept can be used with memory. As our brains work, we consume energy and generate heat. Heat generation increases entropy and is irreversible. Even though our brains become more ordered due to memories being pushed into it, the overall entropy of the system increases. That is why we have the notion of time, as one memory gets pushed into our brains after another. Indeed, the psychological arrow of time have to align with the thermodynamic one.
We can think of our lives as a railway track, each event disconnected from the rest but for the tracks of our memories. I ate dinner then brushed my teeth, but who is to say that one happened before the other? The only reason why I feel that way is due to my memory. For a fourth-dimensional being, it is entirely possible that I brushed my teeth before I ate, or I brushed my teeth while I was eating. Rather, for a fourth-dimensional being, there should be no notion of time at all, as everything that has being or will ever be exists at the same instant. That is a topic for another day, as it is getting late and I should sleep.