The Three Signs
When God spoke to Moshe at the burning bush, Moshe was concerned that klal Yisrael, crushed by the terrors of the Egyptian slavery, would not believe that he was to be the agent of God who would bring about their redemption:
Moshe answered and said, "But they won't believe me, and they won't listen to my voice, for they will say that God did not appear to you." (Shemos 4:1)
God thus provided Moshe with three signs, miracles he could perform at will before the people to convince them that he really was the man to lead them out of Egypt. The first sign was a stick that Moshe would throw to the ground, where it would change into a serpent. When he grasped the serpent by the tail, it reverted to a stick. The second was that when Moshe thrust his hand into his breast, it contracted leprosy. When he returned it to his breast, it reverted to its healthy state. Finally, Moshe was to take some of the waters of the Nile and pour it onto dry land, where it would become blood.
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Before analyzing these signs in any depth, let us consider for a moment why klal Yisrael needed them to convince them of Moshe's authenticity. Why couldn't they simply believe that he was the redeemer without this proof? We may first of all rule out the possibility that klal Yisrael doubted that God had the power to redeem them from their oppressors and thus needed the signs to prove that He was capable. This is highly improbable, as they had a tradition from their ancestors that God was omnipresent and powerful and would one day redeem them. In reality, their disbelief was prompted by a much deeper and personal motive. They did not feel ready for redemption; they knew that the exile had a specific aim, which they believed had not yet been realized.
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THREE SINS, THREE GENERATIONS
To appreciate the underlying function of the Egyptian exile, we must go back in human history to three crucial earlier generations. These were the generation of Enosh, that of the Flood, and that of the Tower of Babel. Each of these generations had a particular spiritual function to fulfill: they were each given a specific proclivity for one of the cardinal sins, which they were to overcome, thereby perfecting the world.
In the days of Enosh, idolatry was invented. The Torah tells us little about Enosh, but the Rambam explains that it was in the lifetime of Enosh that people assumed that, since God had appointed the heavenly spheres to control the world, it was appropriate to worship them. From that error onward, the world slid into idolatry. Enosh's contemporaries could have perfected the world by using their powers for good. Instead they used them to introduce idolatry.
The generation of the Flood were sexually immoral. The Torah tells us the reason for their destruction:
The world was corrupt before God... (Bereishis 6:11)
Corrupt - this is the language of sexual immorality and idolatry. (Rashi loc. cit.)
The great evil instigated by this generation was sexual misbehavior. Again, they had a particular proclivity for this sin, could have overcome it and improved the world, but failed.
Finally, the builders of the Tower of Babel are characterized, if not as murderers, at least as people with no regard for the sanctity of human life:
If a person fell down and died, they paid not the slightest attention, but if one brick fell down, they sat down and wept, saying, "Woe to us! When will we bring up another to replace it?" (Pirkei D'Rabbi Eliezer 24)
So these three special generations failed to cleanse the world from the three cardinal sins - idolatry, sexual immorality, and murder - instead introducing and consolidating them.
The consequence of the Tower of Babel incident was that the world was divided into different nations. From this division came the 70 primary nations and Avraham, the forerunner of the Jewish people. The Jews were to be the torchbearers of morality, in some way the conscience of the peoples of the world. As such, the 70 progeny of Yaakov and their descendants had to go into exile in Egypt to rectify the three cardinal sins before they could become the nation of God. They had to undergo a series of oppressive decrees to wipe out even the slightest trace of these sins left in their collective consciousness by the three earlier generations.
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HAD THE TIME ARRIVED?
At the time of Moshe, only slightly under 210 years had elapsed since Yaakov and his family had arrived in Egypt. However, the exile was supposed to last for four hundred years, as God had originally promised to Avraham. The Midrash states:
When he went and said to the Children of Israel, "You will be redeemed in this month," they replied, "Moshe Rabbeinu, how can we be redeemed? Did not God say to Avraham Avinu that the exile would last for 400 years? Only 210 have passed!" (Pesikta Rabbasi 15:8)
This is why Moshe thought that klal Yisrael would not believe that he was the redeemer - they knew that they needed 400 years of slavery to achieve complete rectification for the sins of the three generations of which they were the spiritual inheritors.
In truth, the time had not yet arrived, for indeed the rectification was not complete. But another consideration had become relevant: klal Yisrael were sinking into the spiritual perversion of Egypt; they had reached the 49th level of tumah (impurity). If allowed to deteriorate further, their spiritual status would have become so bad that they would never have been redeemed. So God chose to redeem them at this stage in the exile, despite the fact that 190 years remained. They would succeed, despite the incomplete exile process, in the merit of the fact that they would eventually accept the Torah. Admittedly they still needed to rectify the faults that the exile had not purged, but this was now to be achieved at the Giving of the Torah.
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UNDERSTANDING THE THREE SIGNS
We now appreciate why Moshe feared rejection from klal Yisrael. God thus gave him three signs to perform before them, which would make them realize that He had decided to end the exile at this premature stage.
The stick turning to a serpent pointed to idolatry. It was the snake in the Garden of Eden who first introduced the rebellion against God which lies behind idolatry. The leprous hand corresponded to sexual immorality. We can see this from a punishment promised to immoral women:
Because the daughters of Tzion are proud and walk with outstretched necks and fluttering eyes, walking and floating as they go, and with their feet they spit poison. And God will strike with leprosy the crown of the heads of the daughters of Tzion... (Yeshayahu 3:16-17)
Finally, the blood corresponded to the sin of murder.
Thus, when Moshe performed these miracles, the nation knew that he understood their concern (that their rectification of the three cardinal sins was incomplete) and that God would redeem them nevertheless.
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A SUBTLE DIFFERENCE
There is, if we look closely, a slight difference between the miracle of the blood and those of the serpent and the leprosy. We mentioned that after the stick was transformed into a serpent Moshe turned it back into a stick. Likewise the leprosy - after his hand had become infected, he thrust it back into his breast, restoring it to full health. Not so with the blood. Moshe poured some water on to the ground, where it became and remained blood. We may suggest that the return of a miracle to its original state conveys a great meaning in our context - while there was a problem in that particular subject, it has been rectified, restored to its correct state. So the serpent and leprosy miracles, which symbolized the sins of idolatry and sexual immorality, indicated to klal Yisrael that the rectification had already been achieved in those matters.
Concerning the blood, Rashi tells us:
Egypt worshipped the Nile, so He smote first their god and then they themselves. (Rashi, Shemos 7:17 et. al.)
Blood is the connecting force between the physical and spiritual, between life and death. Indeed, murder is called "bloodshed" - through spilling the blood, the connection between the soul and the body is severed, resulting in death. Once blood is spilled, it cannot be rectified, for we cannot bring the dead back to life. This explains why the blood that Moshe poured on the ground did not revert to water after the miracle was over: the blood that had been spilled and would still be spilled in the downfall of Egypt could not be undone.