According to Heidegger, to say that something "is" meant for the Greeks that something emerges from dark hiding, appears to us, shines before us, and endures or lasts for a while in that appearance and shining. Of course, this appearance, shining and endurance may take place through our senses, but it can also take place in our mind and memory. I want to point out that in this explanation of Heidegger, it becomes clear that "being" is a timebound, historical concept: the moons of Jupiter did not appear to the Greeks of 400 BC, since they first appeared after the invention of the telescope around 1600 AD, and so they had no being for them. Reciprocally, many things had being for the Greeks (their religious experiences, for example) which have no being for us —we have forgotten them.