Embryogenesis

I think the section that begins on pg.19 and runs through the top of pg. 26, with its discovery of two types of repetition, is really the main meat of the introduction.  It falls into two pieces at the beginning of the last paragraph on pg. 23.  The first piece gives some examples of how we can see something going on under the repetition of the same form; the second piece treats the idea of a hidden repetition beneath the outward repetition, of a subject and an object of repetition more generally.

Our problem concerns the essence of repetition. It is a question of knowing why repetition cannot be explained by the form of identity in concepts or representations; in what sense it demands a superior 'positive' principle. This enquiry must embrace all the concepts of nature and freedom. Consider, on the border between these two cases, the repetition of a decorative motif: a figure is reproduced, while the concept remains absolutely identical ... . However, this is not how artists proceed in reality. They do not juxtapose instances of the figure, but rather each time combine an element of one instance with another element of a following instance.

I've added some emphasis to that paragraph that's not in the original because I found it tough to understand the original without explicitly understanding why those phrases were there.  How is the repetition of a decorative motif "on the border" between "concepts of nature and freedom"?  And why is Kadiweu face painting the first example we get?  I only made sense of this passage when I flashed on the idea that the "artist" here is not necessarily a human one.  Birds and bees and nature in general produce plenty of "repeating decorative motifs", and so do the Caduveo, the muslims, and M.C. Escher.  Everything from birdsong to a hexagonal nest to the zebra's stripes to the drawings that might illustrate those fit here.  Hence the example can be considered either as part of nature or part of freedom (both human and animal), depending on who you take to be the artist at work.  The perfect illustration of this is naturally some indigenous art that we industrialists study as a way of understanding our own seemingly lost connection to a natural world.

With this reading in mind, you can choose something like "an organism with a symmetrical left and right side" to be the motivation for the following part:

These remarks stand for the notion of causality in general. For it is not the elements of symmetry present which matter for artistic or natural causality, but those which are missing and are not in the cause; what matters is the possibility of the cause having less symmetry than the effect. Moreover, causality would remain eternally conjectural, a simple logical category, if that possibility were not at some moment or other effectively fulfilled.

I think Deleuze is thinking specifically of embryogenesis here, an idea I know is close to his heart.  Consider the egg you once were.  Ahhh, the good old days.  You started off as an undifferentiated oval.  Yet somehow, though a complicated process of chemical signaling that resulted in gradual step-by-step differentiation, you ended up with a left and a right, a top and a bottom, a front and a back.  If we just focus on the left-right symmetry that results, we can easily see what may literally be our very model for "the same" being repeated -- having two hands and feet.  And I think the Deleuze is even suggesting here that bi-lateral symmetry may provide our deepest model for causality.  Consider Hume's objection to the concept of causality -- it has to be more than just regular and repeated conjunction.  Okay, fair enough, but how about regularity and repetition on display at the same time?  Does looking at your hands put the idea of causality into your brain to begin with?  I'm not going to head down Lakoff and Núnez's speculative road here, but it does seem intriguing.

At any rate, what scientists call symmetry-breaking differentiation can become our model for what Deleuze means by repetition.  The important point is not the two identical hands that result from the process, but the cascade of differences that drive the process (beginning apparently with the way cilia on the embryo create a uni-directional flow of extra-embryonic fluid).  Admittedly, he's using "symmetry" here in the opposite sense that scientists use it, but in a way that's much closer to our everyday use.  Most of us don't normally rush to call a circle symmetrical, but draw a couple of dots in it: 😶 and you are halfway to a symmetrical face.  Mathematically, you've actually reduced the symmetry of the object by adding the eyes because you can no longer recover the same shape when you rotate it 90 degrees: but, kinda interestingly, most folks will immediately tell you that the first face is symmetrical and will hesitate on the second, and on the original circle.

And now I think we're really cooking with gas, metaphorically speaking.  I suspect that embryogenesis is going to be the core metaphor of the book because it illustrates the way that the identity of repeated forms is produced from a cascade of differences.  In other words, repetition is difference, and the difference of differences, and it's turtles all the way down.  

The negative expression 'lack of symmetry' should not mislead us: it indicates the origin and positivity of the causal process. It is positivity itself. For us, as the example of the decorative motif suggests, it is essential to break down the notion of causality in order to distinguish two types of repetition: one which concerns only the overall, abstract effect, and the other which concerns the acting cause.

Instead of just understanding repetition categorically as a process, we can now point to a specific process that illustrates what we're talking about.   Eggs, seeds, and God are the original rhizome.  Yeah, God.  But we'll have to come back to that.