If your brother grow poor, and his means fail, you shall support him [with a loan]... Do not take interest and usury from him... I am G-d your G-d, who has taken you out of the land of Egypt of Egypt... to be your G-d 
-Leviticus 25:35-38

From these [concluding words], our sages have derived: One who accepts upon himself the prohibition of usury, accepts upon himself the yoke of Heaven; but one who rejects the prohibition of usury, rejects the yoke of Heaven 
-Sifra, ibid

The Torah strictly forbids the collection or payment of usury on a loan granted from one Jew to another. However, there is a procedure, called heter iska ("partnership clause"), by which it is permitted to profit from funds extended to one's fellow. In a heter iska contract it is stipulated that the money is not a loan but an investment in a joint business venture, whose profits are to be shared between the owner of the capital and the one who has been granted the right to use it and deal with it.

Why is interest on a loan forbidden while profit-sharing on an investment is permitted?

The legal difference is that in the case of a loan, the money is no longer the property of the lender: from the moment the borrower receives it, it is his in every respect (it is only that in receiving the loan he assumes the obligation to make a payment in the same amount to the lender at some future date). So if the lender were to collect a fee or percentage in return for the benefit the borrower is deriving from the money, he would be being rewarded for the fact the money was once his, not for something he is contributing now. This the Torah forbids. On the other hand, in the case of a heter iska agreement, the money remains the property of the investor (in partnership with the one to whom the money has been entrusted); the compensation he receives is not "free profit," but profit that his money is currently generating. (This is also the difference between extending a loan in return for interest, which is forbidden, and renting a home or other object to another in return for payment, which is permitted. While loaned money becomes the property of the borrower, a rented object remains the property of its owner.)

Body and Soul
The Zohar states that Torah has both and a body and a soul. The Torah's "body" is its physical dimension: its recounting of the physical history of the universe and its instruction of the physical life of man. Animating this body is a soul---a spiritual dimension, in which every law and event, and their every detail, has its metaphysical significance.

Body and soul complement and fulfill each other. The body is a vehicle for the soul, extending the soul's reach to areas it could not touch on its own; the body of Torah is its soul's implementer, realizing its ethereal concepts as concrete truths in a concrete world. On the other hand, a body without a soul is dark and cold: often, a law or event in Torah might seem dry, prosaic or trivial, until it is viewed in the elucidating light of its spiritual import.

The same applies to the laws of usury and heter iska. Viewed solely in terms of their application to our financial lives, these might seem highly technical, or even pedantic; heter iska sounds like an elaborate loophole by which to circumvent the prohibition of usury. Is there really that much of a difference between these two ways of being rewarded for granting use of one's capital to another-a difference equivalent (as the above-quoted Sifra states) to the difference between accepting the very notion of G-d's authority or rejecting it, G-d forbid? For this we must look to the soul of this law, to the concept behind its material incarnation.

Before and After
Our sages tell us that G-d Himself observes all that He commands us to do. A closer examination of their words reveals that there are, in fact, two aspects to G-d's observance of the mitzvot.

Quoting the verse (Psalms 147:19) "He instructs His words to Jacob, His statutes and His laws to Israel," the Midrash states: "G-d's way is not like the way of flesh and blood. The way of flesh and blood is that he instructs others to do, but does not do so himself; G-d, however, what He Himself does, that is what He tells Israel to do and observe." In other words, the mitzvot originate as divine deeds (His statutes, His laws); then, as a result of the fact that these are "what He Himself does," "He tells Israel to do and observe" them.

On the other hand, other sources imply the reverse: that our observance of the mitzvot causes G-d to respond in kind (e.g. Yalkut Shimoni, Eicha, section 1034 - "One who studies Torah, G-d sits opposite him and studies with him"). Thus, there are two levels to G-d's observance of the mitzvot: the level on which it precedes and enables our observance, and a second level, on which G-d is "moved" to these deeds in response to our doing them.

A mitzvah is an embodiment of the divine will; doing a mitzvah creates a connection (the word mitzvah means both "commandment" and "connection") between man and G-d, between its human implementer and its divine conceiver. This is why G-d must first "do" a mitzvah before we can do it. Creating this connection is obviously beyond the capacity of finite and earth-bound man; it is G-d's initiation of a particular connection that empowers us to do the same.

But why does G-d follow our observance with an observance of His own? Certainly He, the paradigm of independence and perfection, is not "moved" or "affected" by anything - unless He chooses to be affected. Why, then, did G-d desire that our fulfillment of His commandments should stimulate a similar response in Him?

The Empty Pitchfork
The answer to this question lies in another, more general question: why did G-d command us mitzvot at all? Certainly, He does not need anything from us. As Elihu the Bozite says to Job, "If you sin, how have you affected Him? If your transgression are many, what have you done to Him? If you are righteous, what do you give Him? What can He receive from you?" (Job 35:6). So why didn't G-d, who is "benevolent, merciful... bountifully kind" and "good to all His creatures," create a world free of demands and restrictions on its inhabitants?

Because a life free of work and responsibility - a life whose blessings are not earned but handed out without cause or restriction - is a life devoid of the only true joy there is: the joy of achievement. G-d's greatest kindness to us is His "burdening" us with the "yoke of Heaven" - giving us a program for life that we are responsible to uphold, and making our material and spiritual well-being contingent upon it.

But G-d did more than hand down a list of do's and don'ts. For work alone is not enough. Unless work has a function, the worker will derive no satisfaction from it, even if it is amply rewarded. Achievement for the sake of achievement is hollow and unfulfilling -- something objectively significant most be achieved.

The previous Lubavitcher Rebbe, Rabbi Yosef Yitzchak Schneersohn, illustrated this point with the following parable: A nobleman was touring his estate and came upon a peasant pitching hay. The nobleman was fascinated by the flowing motions of the peasant's arms and the graceful sweep of the pitchfork through the air. He so greatly enjoyed the spectacle that he struck a deal with the peasant: for ten rubles a day, the peasant would model his hay-pitching technique in the nobleman's drawing room.

The next day, the peasant arrived at the mansion, hardly concealing his glee at his new line of "work." After swinging his empty pitchfork for an hour, he collected his ten rubles---many times his usual take for a week of backbreaking labor. But by the following day, his enthusiasm had somewhat abated. Several days later he announced to his master that he is quitting his new employment.

Said the nobleman: "I don't understand. Why would you rather labor outdoors, in the winter cold and summer heat, when you can perform such an effortless task in the comfort of my home and earn many times your usual pay?"

"But master," said the peasant, "I don't see the work."

So to grant meaning and fulfillment to our lives, G-d, made that our every action should have an objective significance - that they should affect Him. Indeed, this is the only objectively significant effect there can be, since G-d is the only objective reality-anty other objective is, by definition, contrived and artificial.

And to extend this significance to every aspect and detail of our deeds, He made that our should effect Him in a manner that reflects their particular earthly nature and function. He made that when we put on tefillin, a mitzvah whose function is to underscore the mind's guidance of and involvement with the emotions, this should cause Him, too, to put on tefillin-cause the involvement of the divine "mind" with the divine "emotional" attributes. The same is true of all 613 mitzvot of the Torah: each has a corresponding impact on the divine reality.

Life is thus neither a humiliatingly free lunch, nor an empty pitchfork pitching imaginary hay, but true "work" - work that earns the blessings it generates and has a true impact and effect, beyond its occupation and rewarding of the worker.

G-d's Money
Thus, the prohibition against usury embodies the very concept of the "yoke of Heaven" - of G-d's imparting true work and achievement to our lives.

If G-d's observance of the mitzvot were only to precede observance, but not also result of it, our relationship with Him would be as a usury-paying borrower. G-d does the mitzvah, granting us the ability to do the same, but there His involvement ceases. The "capital" is now wholly in our domain, our efforts at His behest disconnected from anything that is truly His - we are only making a "payment" in return for what He has given us, like a borrower who pays the lender for the fact that he has extended him a loan.

But G-d's contract with us is not that of a usurious money-lender, but of a heter iska investor. He extends us the capital and insists on payment in return, but He stresses that this is a partnership, an ongoing involvement. He retains His stake in the capital throughout our use of it, being affected, as we are, by the ups and downs of the market of our lives.

G-d desired that we emulate His relationship with us in our relationship with our fellows - that the nature of the "profit" He exacts from His creation should dictate the manner in which we profit from what we extend to one who has need for the resources in our possession. One who disregards the prohibition of usury, rejects the divine partnership in his life, a parnership that makes our every endeavor a true and fulfilling acheivement.