But God had also promised to give Abraham many descendants. And now Abraham is 75 years old, and Sarah is over 65, and they are childless. He needs to know. Abraham expresses his doubts, and he asks for clarification. He needed to know with certainty what God meant by His promise.
Clarification is for the mind because our understanding is weak.
Language can be ambiguous. Words can have different meanings. There can be equivocation. But I am childless. And according to custom, Eliezer of Damascus, my servant born in my house, is my heir.