Parashat Vaera: The Plague of Blood

Question: "The sorcerers of Egypt did the same by means of incantations, and Pharaoh's heart was strong and he did not listen to them, as Hashem had spoken" (Shemot 7:22). Why did Hashem choose to bring plagues which the sorcerers knew how to perform?
Answer: Some explain that this is exactly how Hashem hardened Pharaoh's heart by giving him the chance to think that Hashem did not perform this act, and that Moshe Rabbenu was a sorcerer. In this way, he truly did not take away his free choice. Similarly, the fact that each plague ended after a specific time was in order to provide Pharaoh with the opportunity to err as to whether it was the result of sorcery or a natural phenomenon which ended. Thus, Pharaoh was given free choice, and Hashem hardened his heart. This is like the opinion of Rabbi Yosef Albo in Sefer Ha-Ikarim, that hardening Pharaoh's heart was providing him with the opportunity to err. And Rabbi Yitzchak of Volozhin explained homiletically: The wonder of drowning humanity in a valley of blood, non-Jewish magicians always knew how to perform this.