After having told Moses to count the Levite clan of Kehat, G‑d instructed Moses to count the Levite clan of Gershon.
Avoiding the Negative; Pursuing the Positive
נָשֹׂא אֶת רֹאשׁ בְּנֵי גֵרְשׁוֹן וגו': (במדבר ד:כב)
[G‑d told Moses,] “Take a census of the clan of Gershon.” Numbers 4:22

There are two steps in preparing one’s home for an esteemed guest. First the rooms are scrubbed clean; then they are decorated with beautiful furnishings and works of art. The same two steps apply to how we make our lives and ourselves into a home for G‑d. First we rid our lives of what is negative and undesirable and then we do what is good and right.

The clan of Gershon carried the outer coverings of the Tabernacle, which protected it from undesirable elements. This corresponds to our job of avoiding harmful activities and influences. The clan of Kehat, on the other hand carried the furnishings of the Tabernacle, each of which represents a particular positive attribute and activity.

Just as Gershon was born before Kehat, it is necessary to first cleanse oneself of negative behavior in order to be able to properly pursue good. Nonetheless, the clan of Kehat was counted before that of Gershon, since cleansing oneself of negativity is only a preparation for the true work: pursuing good.1