Then the LORD said to Moses, "Rise up early in the morning and present yourself to Pharaoh, as he goes out to the water, and say to him, 'Thus says the LORD, "Let my people go, that they may serve me. Or else, if you will not let my people go, behold, I will send swarms of flies on you and your servants and your people, and into your houses. And the houses of the Egyptians shall be filled with swarms of flies, and also the ground on which they stand. But on that day I will set apart the land of Goshen, where my people dwell, so that no swarms of flies shall be there, that you may know that I am the LORD in the midst of the earth. Thus I will put a division between my people and your people. Tomorrow this sign shall happen."'" And the LORD did so. There came great swarms of flies into the house of Pharaoh and into his servants' houses. Throughout all the land of Egypt the land was ruined by the swarms of flies.
Exodus 8:20-24
A change now appears in the nature of the judgment plagues. Up to this point they had afflicted the entire land, and the children of Israel suffered with the Egyptians in a common affliction; but now, the land of Goshen was excluded from the visitation and the Israelites were given immunity. This was surely designed to demonstrate that the series of plagues could not be attributed to chance circumstances. This is a very necessary exercise; there is nothing more impressive than the apparent inability on the part of those under divine discipline to perceive the hand of God at work in the misfortunes and afflictions that come upon them. It would be stretching credulity too far to suppose that these plagues, following one after the other upon Moses' threatenings to Pharaoh, should have been mere coincidence, but there is an incorrigible part of human nature that is prepared to believe even that. And so the real meaning of the plagues is placed beyond the shadow of a doubt with the miraculous protection afforded the Israelites in Goshen. The force of this irresistible logic-in-action seems to have come home to Pharaoh; but seeing the truth is one thing, obeying it is quite another, and the king's further reaction, as we shall see in the next Note, amply demonstrates the fact that stark rebellion as well as ignorance and blindness lies at the heart of human sin. Pharaoh had begun by asking, 'Who is the Lord, that I should obey His voice?', and now, having learned Who He is, he is found to be implacably opposed to paying heed to His voice. Sin is at last unveiled in him as being exceeding sinful.