(Exodus 31:13) says: "For it is a sign between Me and you." The Mekhilta reads "between Me and you" — between God and Israel — to the exclusion of the other nations. The Sabbath is a sign that belongs uniquely to the relationship between God and the Jewish people. It is not shared with the peoples of the world.
"For your generations" — the Sabbath must be observed throughout all generations. It is not a temporary measure or a time-limited covenant. It persists as long as Israel persists.
"To know that I, the Lord, sanctify you" — the Mekhilta asks what this adds. From (Exodus 31:16) — "And the children of Israel shall keep the Sabbath" — we already know the obligation exists. Perhaps "keep the Sabbath" applies to everyone in the household, including a deaf-mute, an imbecile, or a minor?
"To know" corrects this. The word "to know" implies conscious awareness. The obligation falls only on those who have the capacity to know — to understand what the Sabbath means and to experience its sanctity intentionally. A person who lacks the mental capacity to "know that I, the Lord, sanctify you" is not bound by the commandment.
This ruling protects vulnerable individuals. It does not exclude them from the Sabbath's benefits but releases them from its obligations. The Sabbath requires not just behavioral compliance but conscious awareness of its divine source. Where that awareness is impossible, the obligation does not attach.