Rebbi — Rabbi Yehudah HaNasi — noticed that the Torah prohibits coveting in two separate places using two different Hebrew words. (Exodus 20:14) says "You shall not covet," while (Deuteronomy 5:18) says "You shall not desire." These are not synonyms. They represent two distinct prohibitions.
The first prohibition — "do not desire" — forbids the internal act of wanting what belongs to another person. The second — "do not covet" — forbids the external act of pursuing and acquiring what you have desired. Together they create a chain: first comes desire in the heart, then comes the active scheming to obtain the desired object.
Rebbi made the causal connection explicit: if one desires, in the end he will covet. The internal feeling inevitably leads to external action. Therefore the Torah's real instruction is: "Do not desire, and you will not covet." Cut off the sin at its root — in the mind — and you will never reach the stage of taking action.
The prophet Micah confirmed this progression. (Micah 2:2): "And they will covet fields and steal them." Coveting leads to theft. The path from thought to crime is short and well-worn. The Mekhilta's teaching maps a psychological descent in three stages — desire, coveting, and theft — and insists that the Torah intervenes at the earliest possible point: the moment a person first looks at what belongs to someone else and thinks, "I want that."