[In the process of Creation] the first elements to be rectified were the refuse and "shells", just as we see that the shell develops before the fruit.

Thus it is written, "Let the water swarm with living creatures" (Gen. 1:20), and similarly the creation of all other aspects of life, until finally, on the sixth day, man was created - the food [inside the shell].

It is for this reason that separating the unwanted from the desired is forbidden on the Shabbat, for this is the way the refinement of the world [in the process of creation] occurred.

The creation of the physical and biological systems of the earth serve as the backdrop to the creation of man, the purpose of it all and through whom all these other elements find meaning.

One of the 39 forms of work prohibited on the Shabbat is "separating" or "refining" [in Hebrew, "borer"], in the sense of separating and removing the undesirable aspects of something, leaving it purified from its dross or chaff. We are taught that the prohibition consists only of separating the unwanted from the wanted, not the wanted from the unwanted. For example, if there is a bowl of fruit in front of us containing both good and rotten apples, we may not, on the Shabbat, separate out the rotten apples in order to leave a bowl of good ones. Rather, we must select out the good ones (which we may then place in another bowl), leaving a bowl of rotten ones. The progression of Creation in the cosmic realm is descending, while on the earthly level it is ascending...

This is because separating the bad from the good is a weekday type of work, i.e. one of the types of work the characterized the six days of Creation. As we see here, as the worlds were created, one after another, the coarsest refuse of the world of Tohu was separated out of the entire mass of sparks.

In the six days of Creation of this world, G‑d first created the lower types of creatures, the mineral world. To do this He had to separate out the lowest, coarsest elements from the creation-soup He had to work with from the first day. The creation-soup left was thus "purified" from all the elements so coarse that only the mineral kingdom could be created from them. Next, G‑d separated out the coarsest elements remaining in the creation-soup, which resulted in the creation of the vegetable kingdom. The creation-soup was thus left purified of those elements so coarse that only vegetative life could be created out of them. Next G‑d separated out the animal elements, and finally, the choicest part left was the human.

(Notice that in both cases the bad is being separated from the good; the progression of Creation in the cosmic realm is descending, while on the earthly level it is ascending.)

Separating the wanted from the unwanted, however, is permitted [even on the Shabbat], since this is not the way the world was created.

Separating this way does not therefore reflect the process of Creation, which of course did not occur on the Shabbat. It is therefore not considered "weekday" work, and is permitted on the Shabbat.

[Translated and adapted by Rabbi Moshe Wisnefsky from Likutei Torah; subsequently published in "Apples From the Orchard."]