Sara Imari Walker is a physicist, (or more properly, an astrophysicist, or even more properly, an astrobiologist since she is looking for “life” in the universe) who has written a book to introduce “assembly theory” to a wider public: Life as No One Knows It: The Physics of Life’s Emergence (Riverhead Books, 2024).  I was drawn to read it because a superficial notice about it in the New Yorker suggested that her positions aligned to some degree with the issues I tangled with in my two recent posts on “Consciousness and Life.”  (Links: https://jzmcgowan.com/2024/12/05/consciousness-and-life/ and

https://jzmcgowan.com/2024/12/13/consciousness-and-life-response-and-clarification/).

Reading the book showed that those links were not all that substantial.  But Walker does declare outright that “to this day . . . we cannot derive life from the known laws of physics, even if we are pretty sure it must be consistent with them” (21).  Hence, she shares my view that physics as the science of matter cannot account for life—which leads us to biology, or biochemistry, as the appropriate sciences if we want to get a handle on life.

Interestingly, because “life” stumps physics, there are various physicists who claim life does not exist: since “modern science has taught us that life is not a property of matter” (6), the very category of life is a mistake, “not a natural kind” (22), but a figment of human thought that doesn’t map onto the way the world really is.  The parallel with those who declare consciousness an “illusion” is fairly direct.  Faced with something we can’t account for within our current scientific paradigms, some just insist those unaccounted-for somethings are not real. 

Walker, instead, thinks the available paradigms are insufficient, not that the data (the fact that life exists) should be discounted.  Her book is going to introduce the “new paradigm” she and her colleagues are attempting to put into place.  That paradigm is called “Assembly theory.”

Before diving into that theory, I must applaud Walker’s quick, but sharp, dismissal of panpsychism.  She describes the panpsychist position succinctly: “perhaps consciousness is fundamental, and therefore all matter is conscious” (40).  Her dismissal is just as succinct: “an easy way to kill two hard problems [i.e. the nature of matter and the nature of consciousness] with one stone is to make the unexplained thing fundamental” (41-2).  Moving the counters around is not a solution (or explanation), but just a way to duck the problem.

So what does “an explanation” look like?  “[B]etter explanations are those that explain more observations, change surprising facts into a matter of course, yield accurate predictions of what one should and should not observe, are falsifiable, rest on relatively few assumptions, and are hard to vary such that changing the details dramatically changes the predictions” (152).  It doesn’t help that Walker uses the word “explain” in this definition.  What does it mean to “explain more observations”?  Usually, I dare say the notion of “explain” is linked to some designation of the causal processes that bring the observed thing into existence.  And that does seem to be what “assembly theory” attempts to do.  I will get to that.  In the meantime, we can register Walker’s assertion that “scientific revolutions and paradigm shifts are driven by new explanations, not necessarily new evidence” (152). The observables are there already (in some cases), we just lack a good explanation of them.  Such, Walker argues, is the case for “life.”  But we should note that, in other cases, there are new observables because new technologies of perception (microscopes, telescopes etc.) bring new objects into view.

Walker’s approach is not to ask “what life is,” but to ask “what life can do”?  And that approach starts from consciousness.  “Here we are not measuring whether something has experience (what consciousness is), but instead whether something that has experience can do different things because it has an internal world (what consciousness does)” (45).  “Does anything in the universe exist that might not be possible if subjective inner worlds did not exist?” (45).  The syntax is tortured here, but the question is whether there are existing things that could not exist if consciousness did not exist.  Walker’s answer to this question is a resounding Yes. Everything in the built world—cities, technologies, books—depends on humans imagining those things first (in some kind of embodied thought space) and then doing the work of constructing them.  “Some things that exist are imagined through abstraction (are counterfactual) and become physical (made actual) through a phenomenon deeply connected to what we call consciousness.  It is not that all matter is conscious, but that consciousness is potentially a window into the mechanism for bringing specific configurations of matter into existence across time.  If this conjecture is true, consciousness creates the possibility for things to exist that otherwise couldn’t because they did not exist in the past” (47-8).  “The key feature is the ability to imagine or represent things that do not exist, such that the act of imagination becomes causal to the existence of some objects” (73).

Construction is, as its name suggests, central to assembly theory.  What Walker wants to locate in the universe is causal power (my term, not hers), or to be more Promethean about it, creative power.  Basically, she is going to make the same claim about “life” that she makes about consciousness.  “Life is the only thing in the universe that can make objects that are composed of many, unique, recursively constructed parts” (90).  The word “many” is crucial to this description of life.  There are simple objects in the universe, ones that are “prebiotic” (her term); that is, they existed before the emergence of life—and, in some cases, still exist in their simple a-biotic (my term) way.  These a-biotic objects are not “life” because they do not possess the capacity to generate new objects. Also, they are generally not complex. Living objects contain many parts. (It seems, although I am not sure I understood this correctly, that Walker tells us that only objects that are comprised of at least 15 different biochemical components cross the threshold over into “life.”)

Walker begins from a variant of a traditional philosophical conundrum: why is there something rather than nothing?  Her variant is: why does the universe contain these specific objects instead of the many other possible objects that do not exist?  Why possible in this posing of the question?  Because there are molecular combinations that do not exist even though they, according to the laws of physics, could exist.  In fact, humans have managed to create some of these non-existants in the lab.  Humans have added to what nature produced on its own. So there are more possible objects than actually existing ones.

So: what caused some objects to come into existence and others not?  The causal mechanism Walker turns to is no surprise: evolution and selection.  Evolution here is doing its usual work, producing random variations over time as organisms reproduce.  Coming into existence takes place over long stretches of time.  It is not clear to me how radically Walker wants to upset the idea that all the matter that ever existed or will ever exist was present from the very start.  But she does want to insist that “time” is an intrinsic component of (living?) matter, not just the stage upon which matter does its thing.  (My “living” with the question mark indicates I am not sure if she is saying “time” is an intrinsic property of all matter, or only of living matter.  I am clear that she does divide matter into that which has life and that which does not.)  In any case, evolution over time means the emergence of new forms of matter, including forms we would designate as “living.” Life is not there from the beginning, but emerges somewhere down the line.  But what emerges as evolution unfolds sets up a variety of constraints; some possible objects become very improbable, close to impossible, once evolution produces a different group of possible objects.  The chain of causes is pretty determinative (even if there is always some randomness in reproduction).

Evolution, thus, produces variants (but within fairly predictable ranges once things are fairly launched), just as it does in modern-day (i.e. genetically informed) Darwinian theory.  More novel is Walker’s understanding of “selection.”  Assembly theory asserts that objects that are “alive” are too complex to simply emerge as products of random genetic mutations, or through any other random physical process.  “Some objects require information—an algorithm—to make them.  These objects will never spontaneously form and must always be constructed via selection and evolution. . . . All objects that require information to specify their existence constitute ‘life’” (146).  “Complex objects, such as molecules, can come into existence only if there is something that can build them reliably, whether it is a cell, an environment, or an intelligent agent. These objects require an algorithmic process to assemble them.  Assembly theory considers the algorithm to be an intrinsic property of the object, rather than a feature of the machine that outputs it” (143).  [Sidenote: Walker does seem to reproduce the very error she mocks the panpsychists for making; she takes the causal mechanisms she needs, namely time and the informational algorithm, and make them “intrinsic” to matter.]

Selection, then, is made by the algorithm that informs (quite literally) the reproduction of the object—or, maybe it is better to say the persistence of the object over time.  It is not the individual who possesses life so much as it is the “lineage” of information that causes the continuity of life forms over time.  This shift in locus can be illustrated even in the case of the individual human being.  “Over your lifetime you are alive because you are constantly reconstructing yourself—what persists in the informational pattern over time, not the matter (at least not in the traditional sense of the word ‘matter’). . . The fundamental unit of life is not the cell, nor the individual, but the lineage of information propagating across space and time” (150).

Life is a process—a process of regeneration where the information to bring new forms into existence (or to continually reproduce existing forms that substitute in for prior ones) plays the role of selection.  Without that information’s causal power, no life.  It is clear that, in Walker’s view, information is necessary to the existence of life, is, in fact, the distinctive “marker” of life.  But whether information is also sufficient to the existence of life is less clear.  Presumably, there is physical “stuff,” the “objects” she keeps talking about.  Information, it would seem, needs to be embodied.  There has to be “matter” for information to be “intrinsic” to.  I don’t think Walker would deny this.  She wants to retain all the laws of current-day physics; she just wants to supplement its accounts of matter and causation with this addition of information as a causal agent embedded in matter.  She is not such a complete “process theorist” as to deny any “objectivity” to objects (as William James and Alfred Whitehead at times approach doing.)  Or if she is that radical a process theorist, her continual (unexamined) talk of “objects” (possible and existing) undermines that radicalism.  Life in her view, it is clear, is a continual making and unmaking, but some of the made things have a relatively stable existence for at least some duration.

In sum, the basic innovation of assembly theory, its supplement to contemporary physics, is the claim that information is causal—and that we cannot explain life without seeing information as its basic cause.  There are secondary innovations about how “life” designates complex objects that are “assembled” out of earlier existing objects which embody the information required to construct the new forms.  That view opens up vistas of novelty and creativity a more straight-jacketed physics might deny.  As usual, the precise biochemistry of information’s creativity is not specified—just as the neural correlates of consciousness continue to elude researchers.  Like panpsychism, assembly theory works to animate matter, to introduce principles of motion within matter that do not reduce to the laws of gravity, acceleration, and entropy found in standard physics. 

I have left out of my account, Walker’s interest in finding out if life exists in worlds (planets) other than Earth.  That’s her astrobiology hat.  I will confess to having little interest in that question.  Life here on earth is more (in fact, too much!) than enough for me.  But to offer a quick and dirty summary of her position (especially since it explains the title of her book, which from my summary of it thus far would be utterly mysterious): since the emergence of life on this planet followed a determinant path set out by the earliest moves in the game, there is no reason to believe that life outside Earth would follow a similar path.  Thus, looking for “signatures” of life on Mars (or anywhere else), such as water or oxygen or amino acids, is the wrong way to go.  Life on other planets might very well have developed from completely different material bases.  The key is informationally driven reproductive processes, not specific molecules or elements.  So “life” elsewhere might differ radically from “life as we know it.”

How does Walker’s book—and assembly theory more generally—jive with the questions I raised about consciousness and life? Certainly, I have to appreciate someone who is a real scientist asserting that physics does not have a way of addressing the concept (the fact) of living beings.  And Walker, as well, must be counted among the thinkers who is trying to advance new accounts of causation, ones that supplement (at least; perhaps they supplant) traditional mechanistic understandings of cause. Which reinforces my sense (derived from a number of writers) that Darwinian theory does not align with mechanistic (“efficient” in Aristotle’s terms) models of causation–and thus calls for other ways of understanding causation. I also think Walker’s focus on what life and consciousness do (their observable effects) as contrasted to worrying about what they are seems a fruitful and sensible way to proceed. Finally, although she never explicitly says so, I think Walker would agree that consciousness is a feature of “living matter,” not of all matter–and that addressing the puzzle of what life is and does is prior to understanding the nature of consciousness. Understanding life is the best way to make progress in understanding consciousness.