The prohibited work on Shabbat consists of the thirty-nine (39) tasks required to build the Sanctuary. From these tasks and the details of their respective laws, one can learn how a Jew should serve the Creator, since it was through these actions that the Sanctuary was built, and we too, through them, build our personal 'Sanctuary' in the spiritual realm. One of these tasks is lighting a fire. The definition of a punishable act of lighting (for desecrating Shabbat) is ignition with a purpose—to illuminate, to generate heat, or even to produce ashes. Someone who burns a table in their yard is free from punishment since they are only performing an act of destruction and derive no benefit from the burning. However, if they need ashes and the fire is intended to provide them with ashes, it is punishable.
THE APPROPRIATE JOB
From this we learn what the work of 'lighting the fire' consists of in the spiritual work of the soul. Fire in itself is not an objective, and its lighting is not defined as work. The lighting of the fire of the soul deserves to be considered a work and part of the service to the Most High only when it has an objective—it requires ashes.
The fire of man is the flame of his soul, as it is written: "The candle of God is the human soul"3. It is required of the Jew to kindle the fire of his soul, that it be revealed and felt in his heart, until it reaches the level when the soul is 'consumed' ("Calot Hanefesh") - the intense desire of the soul to rise and merge with its infinite Divine source.
ELEVATION IS NOT THE GOAL
A person might think that this elevation is an end in itself. It will ignite the fire of love for Hashem in their soul, and they will be completely immersed in the lofty feelings of desire and passion for the Divine light. Their soul will burn with an intense thirst to detach from the body and cling to the Shekhinah, and all the matters of this world will be in their eyes like absolute nothingness.
The Torah comes and tells us that this is not the true kindling. It is true that it is a pure fire, but it lacks purpose; a fire of this kind is not considered a work through which a Sanctuary for Hashem is built. For this fire to be part of the construction of the Sanctuary, it must be accompanied by a goal in the earthly world; it must "need ashes.".
TRUE ELEVATION
Ash is the most material and coarse element of matter, and that is why it is what remains after the fire. And here we are suggesting that the purpose of fire is precisely ash, the lowest and most elemental matter. Every fiery awakening to the Most High must have a purpose: to strengthen the fulfillment of the Torah and its precepts specifically in concrete action, in this lower world, between the purest elevation and the concrete act of fulfilling the mitzvah.
And indeed there is no contradiction here, since the sublimation and refinement of this lower world are the goal of God's desired Creation. It follows, then, that precisely when the Jew dedicates himself to the refinement of the material world, he rises to the highest level and embodies the Supreme Will of the Most High.
It is specifically this work that leads the world to its goal and thus elevates man to the highest heights, up to the extraordinary elevation that will take place when the true and complete redemption comes at the hands of Mashiach Tzidkeinu.
(Likutei Sichot Volume 36, Page 187)
GRADES:
1. Shabbat 49,b See also 96,b Bavá Kamá 2, end Page 2. Rambam Hilchot Shabbat chap. 12 Halacha 1 3. Mishlei 20:27