Now Joseph reaches the real wall. There is none in the house greater than I, nor hath he restricted me from anything but thyself, because thou art his wife: and how can I do this great wickedness, and become guilty before the Lord? (Genesis 39:9). The Targum Pseudo-Jonathan preserves what the Sages identified as the decisive clause: the one true restriction, and then the escalation from the wrong done to a man to the wrong done before Marya, the Lord.
Bereshit Rabbah 87 hears the architecture of the refusal. Joseph could have stopped at she is his wife. That alone should have been enough to answer. But he adds a further sentence — how can I do this great wickedness, and become guilty before the Lord? The Sages teach that this second sentence is what keeps him intact on the day when the first sentence feels too small to hold.
There will come a day, the midrash says, when Potiphar is far from home and the house is empty (Genesis 39:11). On that day, the obligation to a distant master is a thin thread. What holds Joseph is the second thread — the one tied not to Potiphar but to God. The Aramaic is precise: become guilty before the Lord (me-kadam Marya). The real audience of Joseph's behavior is not the Egyptian officer who is away on business. It is the God of his father Jacob, the God of Isaac, the God of Abraham.
Pseudo-Jonathan, redacted in Eretz Yisrael in the early common era, is quietly teaching a principle: the promises we make only to other humans will fail on the day we are alone. The promises we make before God stand when the house is empty.
The takeaway is the moral center of the Joseph narrative. Fidelity in the presence of a witness is common. Fidelity in the absence of a witness is the beginning of what the tradition calls righteousness. Joseph's two sentences — she is his wife, and how can I sin before the Lord? — are the scaffold he will stand on when there is no one else in the room.