Why are there ten sefirot and not nine or eleven? I can give no reason from the standpoint of formal logic why this should be so, but we can give both Biblical and scientific reasons. From the viewpoint of science, one of the most beautiful theories about the structure of the universe is a model known as "superstring theory". Superstring theory accomplishes an elegant unification of the fundamental forces of nature by postulating the existence of extra dimensions. In particular, superstring theory works only under the assumption that the universe contains either 10 or 26 dimensions. Both of these numbers are significant in Judaism, particularly the latter which is the numerical equivalent of the most sacred name for God. However, the current version of superstring theory suggests that other parts of physics are incompatible with a 26-dimensional universe, and so we are left with 10 dimensions in which to operate. Obviously, an interesting parallel between Kabbalah and physics.
Biblically, the ten sefirot are believed to be alluded to by the ten times in Genesis that the phrase "and God said" appears. It is also stated in Pirkei Avot, Chapter 5, that "With ten divine utterances was the world created." Furthermore, it is relevant that human beings have ten fingers and ten toes since we are seen as being made in the image of God, and thus, our structure should recapitulate more cosmic structures. In a more general sense, the ten sefirot refer to the creation of structure where previously there was none both to give form to the universe and to provide a mechanism for God's interaction with the universe. On a more particular level, the emphasis that there are ten and not nine or eleven is there to tell us something very important. The topmost sefirah, Keter, is a connecting link with God. To deny this sefirah would also be to deny God's presence in the world. The belief in a purely mechanical universe would be the resulting conclusion. On the other hand, to believe in more than ten could be construed either as believing in more than one God in the universe or as believing that God is a sefirah that is finite and limited in nature. The belief of Judaism and Kabbalah, of course, is that there is only one God and both good and evil are ultimately the creations of the one Holy God.
"I form the light, and create darkness; I make peace, and create evil; I the Lord do all these things. -Isaiah 45:7"
The teaching that there are only ten sefirot also tells us that the pseudo-sefirah Daat (Knowledge) is really only a reflection of Keter (crown, pure consciousness) and does not exist independently.
"Understand with wisdom, be wise with understanding" tells us that we must use both halves of our brains in reaching decisions. We must use both our higher wisdom (Chokmah) and our higher understanding (Binah). To understand without wisdom is to pursue science without guidance from a higher spiritual awareness. For example, science is now gaining the ability to easily clone life forms and to produce a variety of genetic manipulations. It is imperative, though, that we let our intuitive side guide us in the correct application of this technology. To do otherwise would be to create a left brain tower of Babel that will ultimately lead to ruin. Similarly, to "be wise with understanding" underscores the need for intellectual development to go hand in hand with our right brain development lest we fall into superstition and folly through a lack of proper discrimination.
"Examine with them and probe from them" conveys two things. First, we should use both halves of our brains in interacting with the world, and second, we should keep ourselves centered in these higher brain functions. In particular since the text explicitly says "probe from them" rather than "probe with them", this suggests that we should center our awareness at a higher level rather than a lower level, and examine the world from such a spiritually elevated perspective.
"Make each thing stand on its essence" instructs us to be aware of the essence contained within all things.
"Regard not the container, but that which is contained therein. -Pirkei Avot, Chapter 4"
In his classic work I-Thou, Martin Buber makes a distinction between relationships where the other person or thing is treated solely as an object, and relationships where the essence of the other person is fully recognized. When we let ourselves become aware of the divinity within another person, we sense that their spark of life is not unlike our spark of life, and that recognition of commonality produces a flow of energy that enhances and changes both of us. However, when we relate to someone or something only as an object, then we have frozen that image in our mind and there is no life within the relationship. Similarly, when we are angry or upset with another, it is hard to perceive their essence.
"Whoever becomes angry, even if greatness has been decreed for him by Heaven, is cast down. -Pesachim 66b"
However, if we take the step to acknowledge their humanity, then the doors to healing through love and compassion are flung open. When another person is out of touch with their own divine nature, it can be hard for us to see it, too. Nevertheless, it is still our obligation to help repair the world by treating them as having been made in the image of God.
"Make the Creator sit on his base" is the final instruction of this verse. The word "sit" suggests a transition from a moving, changeable state to one that is unchanging. Remember that the ultimate nature of the universe is that there is only God. According to the Zohar, the first part of the Shema ("Hear, Oh Israel, the Lord is our God, the Lord is One") emphasizes that all is One, while the words that follow ("Blessed is His Name, glorious is His kingdom, forever and ever") stress the experience of Oneness within diversity. If we do not have this underlying awareness of the Life of all the Worlds within our existence, then we do not experience that changeless fixed point which is at the center of all change. In this case, we experience only the dynamic movements of the power of God within the natural world, and direct awareness of God is concealed from us. However, when we become aware of that changeless unity that accompanies all our perceptions, then we "sit the Creator on his base". From that point on, we can always center ourselves by first becoming aware of the spiritual essence of other people, places, and things, and then by letting that essence point our awareness to the unchangeable spiritual source that underlies all phenomena.