John Gill Commentary Genesis 27:32

"And Isaac his father said unto him, Who art thou? And he said, I am thy son, thy first-born, Esau." — Genesis 27:32 (ASV)

And Isaac his father said to him, who are you ?
&c.] Hearing another voice more like Esau's than what he had heard before surprised him, and therefore in haste puts this question:

and he said, I am your son, your firstborn Esau ;
all which was true in a sense; he was his son, and he was Esau, and he was his firstborn by nature, but not by right, for he had sold his birthright.