Rashi on Leviticus 18:4/רש"י על ויקרא י״ח:ד׳

1את משפטי תעשו YE SHALL DO MY JUDGMENTS — Matters prescribed in the Torah which are in conformity with the human feeling of justice such as one feels ought to be ordained if they had not been already ordained by the Torah.

2ואת חקתי תשמרו AND KEEP MINE ORDINANCES — matters which are decrees of the King (promulgated without any reason being stated) against which the evil inclination raises objections: "Why should we observe them and against which also the nations of the world raise objections, as e. g., the prohibition of eating swine’s flesh, of wearing clothes of a mixture of wool and linen, the purgatory power of "water mingled with the ashes of the Red Heifer" (טהרת מי חטאת) — therefore it is stated: "I", the Lord, have enacted this for you — you are not at liberty to evade the obligation (Yoma 67b).

3ללכת בהם TO WALK IN THEM — do not free yourselves from their environment, i.e. that you must not say, I have acquired Jewish wisdom, now I will go and acquire the wisdom of the other peoples of the world in order to walk in their ways.