From Slime Mold to the Structure of Existence
The most basic living creatures engage in it; human thought is built upon it. But are we discovering the world's structure or just imposing our own?
What does a slime mold searching for food have in common with a scientist categorizing species or a philosopher debating the nature of reality? More than you might think. Classification—the act of sorting our world into distinct groups—is a fundamental ability that extends far beyond human psychology. It is an ancient, biological process that even the most basic living creatures rely on for survival 1 2 .
Even simple organisms like slime molds demonstrate classification behavior when navigating their environment and finding food sources.
Human thought processes are fundamentally built upon categorization, from basic perception to complex scientific taxonomies.
Consider the slime mold, an organism so simple it often exists as a single cell. When food becomes scarce, these cells coalesce into a single body that can navigate a maze. Branches that find food thrive; those that don't, die off. Through this process, the slime mold collectively classifies correct from incorrect turns, demonstrating a rudimentary form of decision-making that brings advantage to the whole 1 2 . This behavior hints at a profound question: Is classification merely a useful tool for survival, or is it a means of uncovering the true structure of reality itself?
This is the central question explored in David S. Oderberg's edited volume, Classifying Reality. This collection of writings delves into the philosophical debate between realism—the idea that the world contains objective categories waiting to be discovered—and conventionalism, which posits that our categories are merely social agreements 1 2 .
As we will see, this debate is not just for philosophers; it shapes how we understand everything from the fundamental particles of nature to our own consciousness.
At the heart of any discussion about classification lies a deep philosophical rift.
Proposes that certain entities and categories exist independently of our minds, conceptual systems, or linguistic practices. For the realist, a tiger is a distinct, objective kind of thing in nature, and our task is to correctly identify the boundaries that reality itself has drawn 1 2 .
Argues that our categories are constructs, constituted primarily by social agreement, language, and human interests. A conventionalist might point out that the way we divide the animal kingdom into species is useful, but ultimately a human invention—a convention 1 2 .
If reality does have an objective structure, what are its most basic components? The late philosopher E.J. Lowe proposed a sophisticated answer to this question in his four-category ontology 1 2 .
Lowe sought to define the minimal number of mutually exclusive fundamental categories needed to describe our world. He proposed that any element in this system must be distinct and incapable of being reduced to a combination of other elements in the ontology 1 2 . His complex system, a revision of formal logic, outlines the kinds of things that form the fundamental components of our world.
| Category | Description | Example |
|---|---|---|
| Substantial Universals | The fundamental kinds or essences that things belong to. | The universal "Tiger" that defines what it is to be a tiger. |
| Non-Substantial Universals | The properties or attributes that things can have. | The universal "Stripedness" (the property of being striped). |
| Substantial Particulars | The individual objects or entities themselves. | A specific, individual tiger. |
| Non-Substantial Particulars | The specific instances of properties in particular objects. | The specific stripe pattern on that individual tiger. |
While philosophers debate the structure of reality, psychologists study the mental processes we use to navigate it. Psychological classification is, in many ways, a special case of the broader process of classification seen throughout nature 1 2 .
However, according to reviewer Paul M. W. Hackett, the psychological study of categories is a neglected area that "lacks a clear theoretical conception of categorizing processes" 1 2 5 . One of the criticisms of Oderberg's volume is that it pays little heed to mereology—the study of part-to-whole and part-to-part relationships—which is crucial for understanding how we build complex categories from simpler components 1 2 .
To address this, Hackett proposes using the mapping sentence, a flexible framework from facet theory, as a tool for incorporating the combined effects of inter-related categories 1 2 . This approach acknowledges that not all classifications are equal and allows researchers to map out the complex relationships between different facets of a concept in a structured way.
| Concept/Tool | Function in Classification Research |
|---|---|
| Four-Category Ontology | Provides a metaphysical foundation for categorizing the fundamental types of entities in reality 1 2 . |
| Mapping Sentence | A psychological tool used to define and relate the facets of a concept within a coherent framework 1 2 . |
| Mereology | The formal study of part-whole relationships, crucial for understanding how complex categories are assembled 1 2 . |
| Realist Approach | A philosophical stance that treats classification as the discovery of objective, mind-independent boundaries in reality 1 2 . |
To see classification in its most basic form, we can return to the astonishing behavior of the slime mold. This is not a thought experiment but a real-world demonstration of a primal, biological classification process.
A slime mold (e.g., Physarum polycephalum) is introduced to a maze.
Food sources are placed at two specific locations within the maze: at the start and at the end point.
The slime mold, which initially spreads its branches throughout the maze, begins to undergo a change.
The branches of the mold that successfully connect to the food source thrive and strengthen. Those that lead to dead-ends or away from the food "die off."
This experiment reveals that the slime mold is capable of classifying correct or incorrect, advantageous or non-advantageous turns in the maze 1 2 . It is a powerful example of how even a simple organism can assess its environment and change its physical form to achieve a goal, a process that can be understood as a behavioral analogue to cognitive classification.
| Experimental Variable | Observation | Scientific Importance |
|---|---|---|
| Food Presence | Acts as a stimulus causing the single-cell organisms to agglomerate into a collective body. | Demonstrates how environmental pressures trigger collective, problem-solving behavior. |
| Path Efficiency | The mold optimizes its shape to form the most efficient path to food, allowing it to solve mazes. | Illustrates a rudimentary, space-searching intelligence and adaptive problem-solving without a central nervous system. |
| Collective Behavior | Single cells act as a collective to identify and reach nutrients. | Challenges traditional boundaries of cognition and suggests that classification is a fundamental biological process. |
Slime mold spreads branches in all directions throughout the maze.
Mold retracts from dead ends, forming the most efficient path to food.
From the maze-solving slime mold to the philosopher's intricate ontology, the act of classification is revealed as a profound and pervasive activity. It is a process that enables survival for the simplest organisms and fuels humanity's most ambitious intellectual pursuits. The debate between realism and conventionalism is not a mere academic exercise; it forces us to question whether the patterns we see in the world are discoveries or inventions.
Oderberg's Classifying Reality offers a compelling argument for realism, suggesting that the world comes with its own inherent structure. Yet, as psychology shows, our own mental tools and mereological relationships play a crucial role in how we build our categories. The challenge, then, is to refine these tools to better mirror the world's true architecture.
The quest to classify reality is ultimately the quest to understand our place within it. It is a journey that connects the behavior of a humble mold to the deepest questions of philosophy, reminding us that the need to sort, to group, and to understand is woven into the very fabric of life.