If a tree casts a shadow is it telling the time?

Russ Abbott

REVIEW

If a tree casts a shadow is it telling the time?

Russ Abbott

Multiagent Systems · Submitted 30 Mar 2006 · 11 references

Pith's verdict: A thought-provoking philosophical essay that reframes Church-Turing as a statement about programs, but the load-bearing fixed/contingent split is stipulated and user-relative, making the central claim hard to pin down.

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arXiv IDcs/0603125 (version 2)

Pith IDBO5576ZY, Pith's permanent record of this paper and its review

CitationRuss Abbott. "If a tree casts a shadow is it telling the time?." arXiv:cs/0603125 (2006). https://pith.science/paper/BO5576ZY

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Claim paper

Why Pith says this

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Editor's note

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Colleague,

This one is a philosophical essay, not a formal result. The interesting move is the reformulation of the Church-Turing thesis: instead of a claim about which functions are computable, it's a claim about which thought processes can, in principle, be expressed as programs. That is a genuine reframing, and it connects naturally to Wegner and Goldin's interaction machines. The paper also earns credit for the clarity of its examples—tree rings, sundials, balance scales—and for being upfront about what it does not know: it explicitly says no one knows how to model multi-scalar environments, and it concedes that its anti-Platonist stance needs more work. That honesty is rare.

The soft spot is exactly where the stress-test note lands. The fixed/contingent split is load-bearing and it is stipulated, not argued. “We can always separate a computational process into its fixed part and its contingent or alterable part” is asserted, and every example is a user-relative partition: the CPU is fixed until you reflash the microcode, the balance scale is fixed until you rebuild it, the laws of nature are fixed until you change your model. Since the user chooses both the partition and the interpretation, the definition can make any physical process into a computation. The paper's response—that a process counts only when we use it to externalize thought—does not rescue this, because “use” and “externalized thought” are doing the work without independent criteria. So the claim risks being unfalsifiable. That is not fatal for a conceptual stance, but the paper sometimes writes as if it is a theorem.

The citation pattern is thin: the key references to multi-scalar entities point back to the author's own unpublished “Emergence Explained,” which is a self-support problem. And the last section on evolution and energy feels bolted on; it does not follow from the definition and could be cut.

Who is this for? Someone thinking about the philosophy of unconventional computing or agent-based models gets a clear, provocative standpoint. It is not a technical contribution. To make it a stronger paper, the author should engage the observer-relative computation literature (Putnam, Searle) and either formalize the fixed/contingent split or explicitly retreat to a weaker claim about what counts as a useful description. With that revision, there is a decent paper here.

Recommendation: send to peer review at a venue that tolerates philosophy of computing, but expect major revision. It deserves a serious referee, not a desk reject.

Referee report

Significance

If the central claim were fully worked out, the paper would offer a genuinely distinctive position on the nature of computation, one that avoids both pan-computationalism and the need for a mind-independent notion of computation. It usefully diagnoses defects in existing definitions and proposes an interpretability-based alternative that connects computation to human thought. The paper is honest about its own gaps, explicitly admitting that the multi-scalar environment is not formally modeled and that the anti-Platonist premise requires further work. Its historical narrative and concrete examples are instructive. The significance is therefore primarily conceptual and programmatic rather than formal; the paper does not deliver a rigorous definition or theorem, but it stakes out a coherent philosophical claim worth further development.

Major objections (4)

[§3 (fixed/contingent split)] The assertion 'we can always separate a computational process into its fixed part and its contingent or alterable part' is load-bearing for the paper's definition, but no criterion for making this split is given. The examples are perspective-dependent: the CPU is called fixed and memory contingent, yet microcode is alterable; the balance scale is fixed and the objects loaded contingent, yet the scale's construction is itself a contingent outcome of manufacturing; the laws of nature are fixed and initial conditions contingent, yet which description counts as a 'law' is a modeling choice. If the user chooses both the partition and the interpretation, the definition becomes doubly subjective and cannot distinguish computation from non-computation. The paper's criticism of Eliasmith's 'alterable rules' as not well defined applies equally to its own notion of a 'contingent environment.' This needs either a formal criterion or an explicit acknowledgement that the split is entirely stipulated, with the consequences for falsifiability drawn.
[§4.2, Abstract] The paper's endorsement of agent-based computing as 'the current consensus view of how we interact with the world' rests on the existence of multi-scalar environments, but the same section states: 'We do not know how to model the multi-scalar face that nature presents to us' and 'we have not been able to develop a formal model of such an environment.' The support for multi-scalar entities is deferred to the author's own unpublished work [4]. This is not an independent argument; it is self-support. If the central conclusion depends on an environment that has no formal model and on an unpublished reference, the conclusion must be flagged as speculative rather than established. The authors should either provide the missing formal framework or substantially weaken the claim.
[§2 (footnote 4)] The anti-Platonist premise that 'ideas exist only within the realm of mental events' is foundational because it underwrites the claim that a physical process becomes a computation only when used to externalize thought. The author acknowledges in footnote 4 that this position 'requires some care' and that 'these issues can be worked out,' but the paper does not work them out. Without this premise, a tree might be instantiating an idea or a computation without any interpreter, which would directly contradict the central claim. Since the premise is neither argued nor defended, the central thesis rests on an unestablished assumption. The paper should either justify the anti-Platonist stance or present the central claim as conditional on it.
[§3.1 (Turing Machines vs. Turing computability)] The revised Church-Turing thesis—'to be considered rigorous a thought process must, at least in principle, be expressible as a software'—is asserted rather than argued. The paper distinguishes computational activities from computable functions and claims that the equivalence of models shows they are equivalent as programming languages, but it does not justify why rigorous thought must correspond to software expressibility, nor what 'software' means at this level of generality. This revised thesis is used later to legitimate the agent-based model, so it is load-bearing. The author should provide a supporting argument or clearly label the claim as a stipulative proposal.

Minor objections (5)

[§3, §3.1] There is a typo in the sentence 'the environment of an fixed process' which should read 'the environment of a fixed process.' Additionally, the section number '3.1' is used twice, for both 'Non-algorithmic computing' and 'Turing Machines vs. Turing computability'; subsequent sections should be renumbered.
[§2.8] The sentence 'We are not arguing that these activates occur sequentially' contains a typo: 'activates' should be 'activities.'
[§2 (footnote 10)] Footnote 10 ends with 'by the Pauli exclusion principle..' with a double period; one period should be removed.
[References] Reference [4], 'Emergence Explained,' is listed as 'to appear in Complexity' and is cited as the source for multi-scalar entities and energy-driven evolution. Since the paper under review is dated 2006 and the reference appears to be unpublished, the author should provide the publication status or a preprint link, and should not rely on it for load-bearing claims.
[§2.4-2.6] The claim that abaci and geometers' tools are 'completely independent of non-human physical processes' is imprecise, since human users are physical agents; the intended contrast is with non-human natural processes, and the wording should be clarified.

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The paper

The paper claims

The paper claims that computation is not intrinsic to nature: a physical process such as a tree's shadow counts as computation only when someone uses it to externalize thought, with a fixed process running against a contingent environment…

The reading

Most attempts to define computation look for intrinsic properties of physical or symbolic systems. This paper argues instead that a physical process becomes a computation only when we use it as a way to work with externalized thought: a tree counting years is just a tree until someone treats its rings as a calendar, and a running computer is only moving electrons until its activity is understood as performing a mental task. Computation is the playing out of one or more fixed processes against a contingent environment whose contingencies we set and interpret. The paper therefore reformulates the Church-Turing thesis as a claim about the expressibility of thoughts as programs, not about which functions are computable. It concludes that agent-based computing in an open, multi-scalar environment is the current consensus model of interacting with the world, and that a formal model of such environments is missing.

What carries the argument

The load-bearing distinction is the fixed-part/contingent-part split: every computational process can be separated into a fixed process (the mechanism, be it a CPU, the laws of nature, or a balance scale) and its contingent environment (the memory contents, the weights on the pans, the shadows falling on a dial). Computation occurs when an interpreter both controls those contingencies and interprets the result as externalized thought. The paper's companion device is the reformulated Church-Turing thesis, which turns the thesis from a statement about the class of computable functions into a statement about programs: a thought process is rigorous only if it can, in principle, be expressed as software.

Extended reading notes

Core claim

The paper's central claim is that computation is not an intrinsic feature of physical processes. A process in nature may be considered a computation only when we use it to work with externalized thought; otherwise it is just a fixed process, such as a tree growing rings or electrons moving in a computer. What makes a computation is the separation of a process into a fixed part (the laws of nature, a CPU, a program being executed) and a contingent or alterable part (the environment, the loaded program, the input), with the practitioner controlling and interpreting the contingent part. From this the paper draws two corollaries: the Church-Turing thesis should be read as asserting that rigorous thought is expressible as software, and all real-world computation is environmentally driven non-algorithmic computing, so an agent situated in an environment behaves like an oracle machine with nature as the oracle. The paper further argues that a formal model of multi-scalar environments is an unsolved problem and that evolution can be understood as the increasing ability of dynamic entities to extract information from their surroundings.

Load-bearing premise

The load-bearing premise is the paper's anti-Platonist stance that ideas exist only in the mind of someone actually thinking them; if ideas could exist independently of minds, a physical process could be computing even when nobody uses or interprets it, and the definitional claim would collapse.

Editorial extensions

If this is right

A tree growing rings, a shadow crossing a dial, and a running computer are each just a fixed process until someone uses them to externalize thought; the same physical event can be a computation in one setting and not in another.
Because the fixed and contingent parts are relative to the interpreter, there is no principled line between program and environment: a loaded program is the environment for a CPU, and data are the environment for a program.
The Church-Turing thesis, restated as program expressibility, implies that a thought process is rigorous only insofar as it can in principle be written as software.
All real-world computation is environmentally driven or non-algorithmic: the focus shifts from the algorithm a process executes to how an environment shapes a process, making an agent in a real environment akin to an oracle machine with nature as the oracle.
Formulating a model of a multi-scalar environment is an open problem; until it is solved, the paper concludes, we cannot formally model how we interact with the world.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

Beyond the paper: if computation is use-dependent, unconventional-computation research is better aimed at building controllable contingencies and interpretation channels than at detecting computational structure already present in nature.
Beyond the paper: the fixed/contingent split is recursive, since what is fixed at one level (a loaded program) becomes contingent at the next (input to that program), suggesting a hierarchy of computational levels rather than one absolute line.
Beyond the paper: the reformulated Church-Turing thesis yields a testable prediction: any reasoning practice that practitioners accept as rigorous but that cannot be expressed as a program would refute it.
Beyond the paper: requiring environments to allow newly arising entities and kinds, as the paper's open-systems view implies, gives a concrete design target for new agent-based simulation frameworks.

Original abstract

Physical processes are computations only when we use them to externalize thought. Computation is the performance of one or more fixed processes within a contingent environment. We reformulate the Church-Turing thesis so that it applies to programs rather than to computability. When suitably formulated agent-based computing in an open, multi-scalar environment represents the current consensus view of how we interact with the world. But we don't know how to formulate multi-scalar environments.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 4,092 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.05 · states limitation 0.77 · human participants 0.06 · controlled comparison 0.02 · randomized assignment 0.01 · states quantitative result 0.02 · compares with prior results 0.07 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v2. Results and the main claim are underlined; show every role.

Physical processes are computations only when we use them to externalize thought. Computation is the performance of one or more fixed processes within a contingent environment. We reformulate the Church-Turing thesis so that it applies to programs rather than to computability. When suitably formulated agent-based computing in an open, multi-scalar environment represents the current consensus view of how we interact with the world. But we don't know how to formulate multi-scalar environments.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#	sentence	role	the authors' own work	states the main conclusion	strength
s0	Physical processes are computations only when we use them to externalize thought.	background 0.82	0.08	0.32	asserted 1.00
s1	Computation is the performance of one or more fixed processes within a contingent environment.	background 0.86	0.11	0.16	asserted 1.00
s2	We reformulate the Church-Turing thesis so that it applies to programs rather than to computability.	contribution 0.98	0.79	0.20	asserted 0.99
s3	When suitably formulated agent-based computing in an open, multi-scalar environment represents the current consensus view of how we interact with the world.	background 0.99	0.06	0.15	asserted 0.97
s4	But we don't know how to formulate multi-scalar environments.	gap 0.82	0.26	0.25	asserted 0.99

What kind of paper the classifier reports this is: position or commentary 0.98 · evidence from argument 1.00.

What was examined, and what was not

sources read

arXiv listing abstract v2

not examined

full text
figures and tables
references and citing sentences
relations to other papers

levels run

P abstract

not run

S sections
T body sentences
C citations
R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

References

Works cited (11)

[4]

Abbott, R., “Emergence Explained,” to appear in Complexity

View on Pith
[1]

IJUC Editorial Board, “Preface [to the first ed ition] from the Editorial Board,” International Journal of Unconventional Computing , (1, 1), pp 1-2, 2004

View on Pith 2004
[2]

Wegner, P. and D. Goldin, “Interaction, Computa bility, and Church’s Thesis,” British Com- puting Journal , 1999

View on Pith 1999
[3]

Google, Help Center , undated. Accessed March 12, 2006: http://mail.google.com/support/bin/answer.py?answer=29433&query=faq&topic=0&type=f

View on Pith 2006
[5]

Albacete, L. “For the Love of God,” New York Times , Feb 3, 2006

View on Pith 2006
[6]

Sowa, J. “Semantic Networks,” http://www.jfsowa.com/pubs/semnet.htm , June 2, 2006, revised from S.C.. Shapiro (ed) Artificial Intelligence-Encyclopedias , John Willy & Sons, Inc, 1992, pp 1493-1511

View on Pith 2006
[7]

Smart Computing, undated. Accessed February 20, 2006: http://www.smartcomputing.com/editorial/dictionary/detail.asp?guid=&searchtype=1&DicI D=18707&RefType=Encyclopedia

View on Pith 2006
[8]

Sowa, J., “Existential Graphs,” 2002. Accessed February 20, 2006: http://www.jfsowa.com/peirce/ms514.htm

View on Pith 2002
[9]

Eliasmith, C, “Computation,” entry in Dictionary of the Philosophy of Mind , C. Eliasmith (ed.). May 11, 2004 < http://artsci.wustl.edu/~philos/MindDict/entry.html>. 10.Dahl, O-J and K. Nygaard, “Simula: an ALGOL-base d simulation language,” Communica- tions of the ACM , (9, 9), 1966, 671-678

View on Pith 2004
[11]

Goldin, D. and P. Wegner, “Behavior and Express iveness of Persistent Turing Machines,” Computer Science Technical Report, Brown University , 1999, http://www.cs.montana.edu/~elser/turing papers/Behavior and Expressiveness of PTM.pdf

View on Pith 1999
[12]

Wegner, P. et. al., “The Role of Agent Interact ion in Models of Computing,” Electronic Notes in Theoretical Computer Science , Elsevier, 141, 2005

View on Pith 2005

Discussion

Disagree with the review, or know something it missed? Leave a comment, highlight any passage above to comment on it, or challenge this review formally.

[1] [4]

Abbott, R., “Emergence Explained,” to appear in Complexity

View on Pith

[2] [1]

IJUC Editorial Board, “Preface [to the first ed ition] from the Editorial Board,” International Journal of Unconventional Computing , (1, 1), pp 1-2, 2004

View on Pith 2004

[3] [2]

Wegner, P. and D. Goldin, “Interaction, Computa bility, and Church’s Thesis,” British Com- puting Journal , 1999

View on Pith 1999

[4] [3]

Google, Help Center , undated. Accessed March 12, 2006: http://mail.google.com/support/bin/answer.py?answer=29433&query=faq&topic=0&type=f

View on Pith 2006

[5] [5]

Albacete, L. “For the Love of God,” New York Times , Feb 3, 2006

View on Pith 2006

[6] [6]

Sowa, J. “Semantic Networks,” http://www.jfsowa.com/pubs/semnet.htm , June 2, 2006, revised from S.C.. Shapiro (ed) Artificial Intelligence-Encyclopedias , John Willy & Sons, Inc, 1992, pp 1493-1511

View on Pith 2006

[7] [7]

Smart Computing, undated. Accessed February 20, 2006: http://www.smartcomputing.com/editorial/dictionary/detail.asp?guid=&searchtype=1&DicI D=18707&RefType=Encyclopedia

View on Pith 2006

[8] [8]

Sowa, J., “Existential Graphs,” 2002. Accessed February 20, 2006: http://www.jfsowa.com/peirce/ms514.htm

View on Pith 2002

[9] [9]

Eliasmith, C, “Computation,” entry in Dictionary of the Philosophy of Mind , C. Eliasmith (ed.). May 11, 2004 < http://artsci.wustl.edu/~philos/MindDict/entry.html>. 10.Dahl, O-J and K. Nygaard, “Simula: an ALGOL-base d simulation language,” Communica- tions of the ACM , (9, 9), 1966, 671-678

View on Pith 2004

[10] [11]

Goldin, D. and P. Wegner, “Behavior and Express iveness of Persistent Turing Machines,” Computer Science Technical Report, Brown University , 1999, http://www.cs.montana.edu/~elser/turing papers/Behavior and Expressiveness of PTM.pdf

View on Pith 1999

[11] [12]

Wegner, P. et. al., “The Role of Agent Interact ion in Models of Computing,” Electronic Notes in Theoretical Computer Science , Elsevier, 141, 2005

View on Pith 2005