Bondage

In parshat Shmot, G-d calls to Moshe on Mount Chorev from the heart of the burning bush. Moshe draws close, and a dialogue ensues whereby G-d sets out several tasks, towards each of which Moshe expresses resistance. One part of this dialogue has G-d telling Moshe that he is to go to b'nei Israel and say to them that He has seen what is going on in Mitzraim, and that He will bring them up to a good, large land, a land flowing with milk and honey.
Moshe responds to this order by saying that b'nei Israel will not believe him. G-d says to this, "What is that in your hand?" G-d then precedes to perform two miracles (and to describe a third) which Moshe will in turn perform for the doubtful b'nei Israel. The miracles are: the rod which turns into a snake and then back into a rod, the healthy hand which becomes leperous and then well again, and the river water which will turn into blood upon striking the ground.
After seeing the miracles, Moshe's comment to G-d is this: "Oh my Lord, I am not a man of words, nor was I yesterday, nor the day before, nor since you have spoken to your servant - because I am of heavy mouth and heavy tongue." G-d's reply is: "Who set for man a mouth? Who will make him dumb or deaf or seeing or blind? Is it not I, Hashem?
The juxtaposition of these psukim suggests that the heaviness of Moshe's mouth was intended, designed, and set within him by G-d. Going with this idea, a question arises: what is the connection between Moshe's "heavy mouth" and his forthcoming role? In other words, why does G-d set for him both a heavy mouth, and the task of leading b'nei Israel out of slavery and into freedom?
My answer to this question begins with a question: what does Moshe mean when he describes himself as being not a man of words, but rather a man of heavy mouth and tongue? Tradition offers a number of explanations - I'll focus on two.
The first is the following well-known midrash (of which there are variations). Baby Moshe is brought into Paroh's court. Placed before him are a dull crown and a glittering coal. The test is designed to determine whether Moshe is an ordinary or extraordinary baby. Paroh's presumption is that if he is the former, he will (like all babies) reach for the irresistible, sparkly coal-- but if he is the latter, he will not be deceived by the lackluster crown; rather, he will understand its significance, and thus reach for it instead. Moshe (extraordinary baby that he is) reaches for the crown, but at that moment, an angel redirects his hand. Moshe ends up grabbing the coal and putting it to his mouth. His tongue gets burned, and he is thereby and forever crippled. The words "heavy mouth" are thus read literally; they refer to a physical disability, a speech impediment.
Another approach is taken by the modern commentator, Umberto Cassuto. Following Ibn Ezra's lead, Cassuto explains that Moshe suffers not from a physical impediment, but rather, from something simpler. He suggests that Moshe does not feel within himself the talents of elocution, and that when he expresses this to G-d, he exaggerates: hence the words "heavy mouth."
I want to go with Cassuto on two points: one, that the words "heavy mouth" do not refer to a physical speech impediment; two, that those words do in fact refer to Moshe's perceived inability to speak publicly with skill. But let's take this reading a little further, and incorporate into it an idea expressed by the midrash. This idea is that Moshe is a crippled man vis-a-vis the task that G-d sets before him. On this reading, it is more than a simple lack of elocutionary skill that is captured by the words "heavy mouth". Rather, with these words, Moshe is expressing a crushing and utterly incapacitating lack of confidence for speaking in front of a crowd.
Now let's look at what's happening on Mount Chorev in light of this reading.
G-d tells Moshe to go to b'nei Israel, and tell them that they will be delivered from slavery into freedom. Essentially, then, Moshe is to tell them that the reality they have known up until now is about to shift, that not only is change possible but it is immanent; the situation they thought was permanent was really only temporary. And G-d has even given to him signs which prove this message very, very literally: an inanimate stick can change into a snake and an animate snake can turn into an inanimate stick; a healthy hand can become diseased and can return to health in a matter of moments; one substance can even change into another. The message of the signs is that reality can shift radically, that drastic change will happen, because no situation is permanent in the face of G-d.
But look at Moshe on Chorev: he's stuck in his stage-fright! Moshe has seen the signs! He's dialoguing with G-d, who is speaking to him from a bush that is burning without being consumed! That miraculous change is possible is a truth that Moshe is learning in living colour! And yet, he is unable to unwind himself from his fear. He's so bound by his lack of confidence that he is incapable of extending the possibility of change such that it includes himself.
At this point in his life, Moshe is very much under the control of his fear. His lack of confidence exercises a mastery over him that he does not have over himself. In other words, Moshe is enslaved on two counts: first of all, he is enslaved to the extent that he would sooner give in to the power of his fear than to serve the One True Master, and secondly, he is enslaved such that he is unable to perceive the world separate from the influence his fear holds over him. Consider this! For Moshe it is easier to adopt an irrational attitude of denial than to believe that change can touch him too! (A stick into a snake? Possible! Slavery into freedom? Possible! Inarticulateness into fluency? No way: "I am not a man of words." Awkwardness into eloquence? Again no way: "I am a man of heavy mouth and heavy tongue.")
I'd like to suggest that the words "heavy mouth" can be read to convey this crippled, bound aspect of Moshe's character. And, recalling G-d's ownership concerning the object of these words, we have returned to the original question: why does G-d set for him Moshe both the condition signified by the words "heavy mouth", and the task of leading b'nei Israel out of slavery and into freedom?
In order to be a successful, empathetic and prepared leader to a nation of slaves, Moshe had to know slavery, and slave mentality, a little bit from the inside. He needed to be familiar with the consequences of unyielding constraint: the crushed and broken will, the distorted and irrational perceptions of what is true, possible, actual. Moshe had to know for himself how hard the progression from slavery to freedom can be, so that he could withstand the long and slow process he was about to initiate.