A certain heathen asked Rabbi Joshua ben Karhah: "Why did the Holy One, the blessed One, speak to Moses out of a thorn bush?" He said to him: "Had it been out of a carob, or out of a sycamore, you would ask the same question. But I cannot send you away empty-handed. Why then out of a thorn bush? To teach you that there is no space free of the divine presence, not even a thorn bush."
Exodus Rabbah 2:9