5. Definition, Nature, and Substance in Natural Philosophy

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Lecture Notes

Main Topics #

The Nature of Definition #

Definition is understood as λόγος (logos)—speech that shows or makes known the nature of a thing. Ammonius Hermaeus defines it as “brief speech showing the nature of a thing” (λόγος σύντομος), which resonates with Shakespeare’s observation that “brevity is the soul of wit.”

In Aristotle’s Categories, the phrase used for a true definition is λόγος τῆς οὐσίας (logos tēs ousias)—“the account of the substance” or what something is.

Three Interchangeable Terms for Essence #

Three words express the essential nature of a thing:

• Nature (φύσις, phusis) - originally meaning birth/nativity, extended to mean the source within, and ultimately the essential character
• Substance (οὐσία, ousia) - from Latin sub-stare (standing under), expressing what is fundamental and subsists by itself
• What it is (τί ἐστι, ti esti) - the direct formulation of essence

Though these terms derive from different roots (birth vs. standing under), they converge in one of their meanings: expressing what something essentially is. This represents a subtle richness in philosophical terminology.

The Fifth Book of the Metaphysics #

Both nature and substance require extended treatment in Aristotle’s Metaphysics Book 5, which is devoted to distinguishing and ordering the meanings of common words used in wisdom. The book is structured around three groups:

1. Names of causes: beginning, cause, element, nature (as beginning/cause of motion), necessity
2. Names of the subject of wisdom: one, being, substance (the chief subject), and their parts (quantity, quality, same, other)
3. Names expressing properties: perfect/imperfect, whole/partial (perfections and defects of being)

Interestingly, nature appears among words signifying causes, while substance appears among words signifying the chief subject of wisdom. Yet both have meanings that intersect at “what it is.”

The Problem of One Universal Method #

The central difficulty Aristotle raises: Is there one method for investigating what things are?

Different sciences define things differently:

• Mathematics defines without sensible matter and without reference to motion
• Natural philosophy must include reference to matter and motion, since nature is defined as “beginning and cause of motion and rest”
• Wisdom (first philosophy) has its own distinctive method

This creates a fundamental problem: if there is no single universal method, then the investigation of essence becomes significantly more difficult, as one must determine “what is the mode in each case.”

The Soul’s Definition as Particularly Difficult #

The soul presents special obstacles to definition:

1. It cannot be studied through external observation alone (unlike artifacts or other natural things)
2. We cannot set up multiple examples of souls to compare, as we can with, say, multiple triangles
3. It is more hidden than other natural substances
4. It may require a hybrid approach to definition if it is a form rather than a complete substance

Key Arguments #

The Difficulty of Knowing the Soul #

Aristotle uses the term πίστις (pistis, belief) rather than absolute knowledge, noting that even achieving this imperfect conviction about the soul’s nature is “in every way a most difficult task.”

This is not defeatism but rather philosophical honesty: the soul is so fundamental and so hidden that complete certitude about what it is may be unattainable, yet even arriving at justified belief is enormously challenging.

Method in Defining: Two Approaches from the Posterior Analytics #

Aristotle identifies two roads to arriving at a definition:

1. Inductive method: Examining multiple examples of the same thing, trying to identify what they have in common (most difficult for the soul, since we cannot easily produce multiple souls for comparison)

2. Divisive method: Finding the genus of a thing, then dividing by differences, adding those differences to the genus. If the resulting speech is convertible with the original thing, a definition is achieved; if not, further division is needed.

Distinguishing Imagination from Thinking #

Berquist explores Aristotle’s distinction between phantasia (imagination) and dianoia (thinking):

Imagination is free; thinking requires reason:

• I can imagine a bottle of champagne without reason or evidence
• I cannot think there is a bottle of champagne without having some ground or reason
• I can imagine a terrorist with a machine gun without believing one is present
• If I think a terrorist is present, I must have evidence (I saw him, the FBI warned me, etc.)

Emotional manifestation of the difference:

• Imagining something does not necessarily produce strong emotion (I can imagine winning $5 million without excitement if I’m very brave)
• Thinking something is so produces the appropriate emotional response (if I think I won $5 million, excitement follows)
• This emotional difference manifests the deeper distinction

The key difference is that thinking involves discourse (λόγος, discursus)—going from one thing to another with rational ground. When I hear the sound of a cat, my mind goes from the sign (the sound) to its cause (the cat being present). This discursive movement from one thing to another is characteristic of thinking and reason, not mere imagination.

Important Definitions #

Definition (logos, definitio): Speech that makes known or shows the nature of a thing; specifically, a brief or concise account of what a thing is. Equivalent to the account of substance (λόγος τῆς οὐσίας).

Nature (physis, φύσις):

• Originally: birth, nativity
• Extended meaning: the source or origin within something
• Scientific meaning: “beginning and cause of motion and rest”
• Ultimate sense (one of several meanings): what a thing is (its essence)

Substance (ousia, οὐσία):

• Literally: that which stands under (from sub-stare)
• Primary meaning: that which exists by itself, not in another
• Secondary meaning (one of several meanings): what a thing is (its essence)

Method (methodos, μέθοδος): Not merely “a way over a road” but something related to a road to be followed; in philosophy, the proper procedure or path for arriving at understanding in a given inquiry.

Examples & Illustrations #

Defining a Cat vs. a Dog #

Berquist illustrates the difference between defining by property and defining by essential difference:

• A cat: “a four-footed animal that meows”
• A dog: “a four-footed animal that barks”

But is something a cat because it meows, or does it meow because it is a cat? Meowing appears to be an effect or property of being a cat, not the essential difference between cats and dogs. One might initially define by sensible properties (meow/bark) because these are manifest to the senses, but a more intrinsic definition (obtained through biology) would be preferable but more difficult to grasp.

Pink Elephants and Unicorns #

Though imagination can present what does not exist, thinking cannot assent to their existence without reason:

• I can imagine a pink elephant, but I don’t think there are pink elephants
• I can imagine a unicorn, but I don’t think unicorns exist in the woods

This shows the fundamental freedom of imagination versus the constraint of rational thought to evidence and grounds.

Questions Addressed #

Is there one universal method for investigating what things are? #

Answer: No. Different sciences must employ different methods suited to their subject matter. Mathematics defines without matter and motion; natural philosophy must include reference to matter and motion; wisdom has its own distinctive approach. This plurality of methods makes the investigation of essence more difficult than it would be if a single universal method existed.

How are ’nature,’ ‘substance,’ and ‘what it is’ related in philosophical discourse? #

Answer: These three terms are largely synonymous in their primary uses, each referring to the essential character of a thing. Though they derive from different etymologies and appear in different groups of meanings in Aristotle’s Metaphysics Book 5, they converge in expressing the essence. This richness of terminology reflects the depth of philosophical thinking about essence.

Why is defining the soul particularly difficult? #

Answer: The soul is hidden from direct observation; we cannot compare multiple souls as we can compare multiple geometric figures; it cannot be defined by external properties alone; and if it is a form rather than a complete substance, it may require a definition that references its relation to the body. Yet even achieving imperfect conviction (pistis) about the soul’s nature is a most difficult task.

What is the relation between imagination and thinking? #

Answer: Imagination is free from the requirement of grounds; I can imagine without reason. Thinking, by contrast, involves discourse—the rational movement from one thing to another—and I cannot think something is so without having grounds or reasons. This difference manifests itself emotionally as well: imagination may produce no emotion, while thinking produces appropriate emotional responses to its content.