To the rear is the commandment:
And you shall not stray after your heart
and after your eyes…. (Numbers 15:39)
This is the mitzvah of guarding one's mind from foreign thoughts and desires. These have the negative effect of diverting one's attention from G-d and confusing his priorities. If one has performed the first five commandments as we have described them, he will naturally strive to protect his precious relationship with G-d from dissolution. Thus, this commandment is the "seal of truth" (emet) (Cf. Shabbat 55a) relative to the previous five, the real measure of his relationship and indication that all of one's pleasure and desire is permeated by the Divine will which he has been attempting to realize as the space encompassing him.
Furthermore, this mitzvah includes the injunction against looking for G-d in "paths" or "trips" other than the Torah. Although these may lure the seeker with the promise of excitement or more immediate spiritual gratification, if he remains true to his ideals he will focus his mind and heart solely on G-d as He has made himself "available," so to speak, through the "path" or lifestyle of Torah. The Torah then teaches one how to properly meet the challenge of finding G-d in all aspects of reality.
Since the foe attacks from the "rear," i.e., tries to catch one off guard, this commandment is placed "behind" the consciousness, as a sort of protective background force-field. Furthermore, "forward" and "backward" are often taken to indicate "the desired" and "the undesired" (or "less-desired"), respectively. If one performs the first five commandments, he is assured in this sixth that the foreign enticements of this world will truly be "behind him," which is why the verse is phrased "And you shall not stray after [i.e. `behind'] your heart and after [i.e. `behind'] your eyes."
If one is truly devoted to G-d, he will desire to make his life productive, or "potent." Rather than wastefully squandering his energy and talents on things that add nothing to– or even detract from–the overall awareness of G-d in the world, he will aspire to imbue all his actions with mindful purpose and drive in fulfilling his Beloved's wish and heightening the world's awareness of Him. The sinfulness of foreign enticements consists in not allowing life to be truly productive.
The order in which we have presented the mitzvot follows their logical order in terms of cause-effect relation, as we have noted: One is first aware that G-d exists; this leads him to deny all other pseudo- divinities. Once that is done he G-d in everything; this leads him to love G-d and his love brings him to fear separation from Him. This fear in turn inspires him to defend himself from distracting or confusing influences.