Time

"People assume that time is a strict progression of cause to effect but actually, from a non-linear, non-subjective viewpoint, it's more like a big ball of wibbly-wobbly, timey-wimey stuff..."

Doctor Who

Since the dawn of humanity itself, generations of humans have ideated many fundamental constructs that have shaped the very fabric of our society. Some of these constructs like mathematics and the sciences are rooted in nature itself while others like racial and national identity are more innately human. Time is a unique construct in that it is not clearly rooted in nature and neither is it unique to humans. Time affects all, from plants to animals to even celestial bodies, and yet, we cannot directly observe time. Time cannot be seen. Time cannot be heard. Time cannot be felt. 

Our entire understanding of time is understood from the indirect observation of this fundamental construct. Time can be perceived through the motion of objects - the movement of some relative to the lack of movement from others. Time can be perceived through the effects of memories - the ability to travel "backwards" in one's life. Time can be perceived through its conflation with physical reality - space-time. With all these avenues for observation, time clearly exists. So what exactly is time? 

There is no good answer other than that time is simply time itself.