Our gemara on Amud Beis discusses the four categories of watchmen:

• The persons who watches for no fee 
• The person who watches for a fee
• The person who borrows
• The person who rents

The first three as described in the Torah, and rabbinically derived from various derashos, bear increasing responsibility commensurate with the amount of benefit derived. Thus, the Free Watchman is only liable for neglect, but not theft. While the Hired Watchman is liable even for many forms of theft and other mishaps, so long as they are due totally to matters out of the watchman’s control.  Finally the Borrower, because he gets all the usage of an owner, is responsible even for many losses that could not have been prevented by any means.

The fourth watchman, who rents an object, is subject to a dispute between Rabbi Yehuda and Rabbi Meir. On the one hand, the Renter ought to be treated as a Borrower, as he also gets all the benefits and usage of ownership. On the other hand, since he pays for these rights, perhaps his fee neutralizes the value of his benefit, and he is entitled to the leniencies of a Free Watchman.  Furthermore, there are two different recorded versions of this dispute, so it is unclear which side of this dispute either Rabbi Yehuda or Rabbi Meir occupy.

The Shalah (Aseres HaDibros, Pesachim, Matzah Ashirah 1) explains a metaphysical dimension to the Four Watchmen.  Each of these watchmen represent a different way of relating to God, similar to the Four Sons of the Haggadah.  The Free Watchman serves God expecting no reward, and therefore is not held liable for certain sins that he commits due to being overtaken (“stolen”) by his impulses; he is only liable for sins of outright negligence. The Hired Watchman represents the person who serves God expecting a reward.  Therefore, he is held liable even for sins that are committed impulsively or compulsively, but still exempt for sins committed that could not have been foreseen or prevented.  The Borrower represents the person who really does not serve God, he does not believe in reward and punishment, and he takes and takes.  Because of that, he is held liable even for sins committed that he could not prevent or stop.  

The Renter represents someone who serves God generally without expectation for reward, but may make a particular mitzvah conditional, such as a person who says, “I am giving this donation to charity on the condition that my son is healed.”  Shalah quotes the Semag who rules that this is permitted, when the person resolves to be happy having given the charity regardless of whether his son is healed; he is merely dedicating the mitzvah as a merit.  However, if his attitude is that “God owes him”, then it is forbidden.  This why the liability of the Renter is represented in the Gemara as an opinion that is variable and fluctuating because the person who performs a particular mitzvah with the hope of a specific merit and benefit (“the Renter”) can be hardly liable as Free Watchman, or heavily liable as Borrower, depending on if his attitude is with a full heart and with only hopes but no demands, or if it is utterly contingent. (In the mystical world there is no such thing as a machlokes, but instead just different dimensions of truth, see Psychology of the Daf Eiruvin 45.)

Mystics in general, and the Shalah specifically will see the physical world and the spiritual is concentric circles and iterations of the same core truths.  The Shalah (Toldos Odom:15) famously states that every word in Hebrew, the holy tongue, is a metaphor or borrowed term from a spiritual reality. Thus, for example, he says the Hebrew word for rain, geshem, is not actually rain. Rather it means the way in which God brings down sustenance and blessings from the upper world to all the lower worlds to allow for growth and development. In this world, rain is the physical manifestation of that, and thus Hebrew uses geshem as a metaphor to represent rain but in spiritual dimensions there also is a form of Geshem. So too, the Four Watchmen in their simplest form, the Torah’s way of regulating human contracts and responsibilities. But those truths about responsibility, once in the Torah, must mean more. In increasingly spiritual dimensions, they represent Man’s responsibilities and covenant with God. 

We are given our bodies and souls on loan from God, which also comes with responsibilities. We must neither neglect this gift, nor treat our relationship and acceptance of this responsibility lightly.