A thing is itself. Sound reasoning begins by respecting what things are.
The Law of Identity is one of the three classical laws of thought. Its fundamental proposition is simple:
A is A. Everything is identical to itself.
Although this may appear self-evident, it establishes something essential to reasoning: we must be able to identify what we are talking about and distinguish it from what it is not.
In On the Plurality of Worlds (1986), philosopher David Lewis writes:
“Everything is identical to itself; nothing is ever identical to anything except itself.”
The principle concerns identity, not similarity. Two things may resemble one another, share characteristics, or belong to the same category without being the same thing.
Why Identity Matters
The Law of Identity provides a foundation for coherent reasoning.
When we make a claim about something, the subject must remain identifiable throughout the argument. We cannot substitute a different subject or change the meaning of a term without recognizing that we have changed what we are discussing.
Aristotle recognized the importance of definite meaning in Metaphysics, Book IV:
“for not to have one meaning is to have no meaning”
His argument was that meaningful reasoning requires terms to have sufficiently definite meanings for us to know what is being asserted.
This has important practical consequences.
Much confusion in business, economics, politics, and everyday reasoning arises when people use the same words to describe different things, or different words to describe what is effectively the same thing.
When meanings shift unnoticed, arguments can appear valid while their conclusions rest on conceptual confusion.
We cannot reason reliably about things we have not adequately identified.
The Practical Move
The Law of Identity suggests a discipline of conceptual precision.
When examining an argument or explanation, ask:
- What exactly are we talking about? Identify the subject clearly.
- What makes it what it is? Establish its relevant defining characteristics.
- Are we confusing identity with similarity? Distinguish the thing itself from things that merely resemble it.
- Are our terms consistent? Ensure that important concepts retain the same meaning throughout the argument.
- Has something changed? Distinguish a change in a thing’s properties from a change in what the argument is referring to.
The Law of Identity does not imply that things cannot change. Nor does it tell us which descriptions of reality are correct. Those questions require additional investigation.
Its value is more fundamental: it provides a logical foundation for keeping our reasoning anchored to identifiable subjects and consistent meanings.
Before we can determine whether something is true about a thing, we must establish what thing we are actually talking about.
References
- Aristotle. Metaphysics, Book IV, Chapter 4, 1006a–1006b.
- Lewis, D. (1986). On the Plurality of Worlds. Blackwell, pp. 192–193.
- Stanford Encyclopedia of Philosophy — Identity.
- Stanford Encyclopedia of Philosophy — Relative Identity.