Ex 4:10-11
10 Moses raised another objection to GOD: "Master, please, I don't talk well. I've never been good with words, neither before nor after you spoke to me. I stutter and stammer."
11 GOD said, "And who do you think made the human mouth? And who makes some mute, some deaf, some sighted, some blind? Isn't it I, GOD?

At first Moses had a hard time because even though he knew that he was talking to God he thought that reality was only what he could see but God is an unseen reality.