physics

Multiple realizability

Multiple realizability is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Multiple realizability rather than just read about it. In short: In the philosophy of mind, multiple realizability is the thesis that the same mental property, state, or event can be implemented by different physical properties, states, or events. Philosophers of mind have used multiple realizability to argue that mental states are not the same as—and cannot be reduced to—physical states.

Key takeaways

Multiple realizability belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Multiple realizability to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Multiple realizability from memory before moving on to harder problems.

Reference excerpt

In the philosophy of mind, multiple realizability is the thesis that the same mental property, state, or event can be implemented by different physical properties, states, or events. Philosophers of mind have used multiple realizability to argue that mental states are not the same as—and cannot be reduced to—physical states. They have also used it to defend or criticize many versions of functionalism, especially machine-state functionalism.

Notion The multiple realizability thesis in the philosophy of mind posits that the same mental state can be realized by different physical states; another way of putting it is that there is a many-to-one mapping from physical states to mental states. Multiple realizability in general is not restricted to the multiple realizability of mental states. Many kinds of things can be realized by numerous physical devices. A wide variety of physical devices can serve as corkscrews, for example. Mental states can also be realized in a variety of ways. Just as the logical states of a Turing machine can be realized by different structural states in different mechanisms, so, by analogy, the mental states of a human being can be realized by different physical states in different individuals. Pain, for example, is correlated with different physical states of the nervous system in different organisms, but the organisms all experience the mental state of "being in pain." Mental states have been claimed to be multiply realizable not only across species and between individuals but also within individuals. At different times, the same individual may realize the same mental states in physically different forms. Neural plasticity—the fact that areas of the brain can assume the functions of other parts that have been damaged as the result of traumatic injury, pathology, natural biological development, or other processes—has long been considered to be an example. But so are more mundane facts about neurophysiology, such as the fact that neurons die and connections between them are rewired. The argument that neural plasticity supports multiple realizability has also been contested. Gualtiero Piccinini differentiates three related properties: variable realizability, multiple realizability, and medium independence.

A property is variably realizable if it can be instantiated by different realizers. For example, both a winged corkscrew and a waiter's corkscrew have the property of removing corks and do so through the same mechanism—a screw and pull mechanism. Because the mechanism is fundamentally unchanged, the property is variably realizable. For a property to be multiply realizable, the property must be able to be instantiated by different realizers and different mechanisms. The classic spring mousetrap and the glue mousetrap instantiate the same property, the ability to trap mice, but they do so through different mechanisms. As such, the property is multiply realizable. A property is medium independent if it can be instantiated by different realizers and different mechanisms and if the inputs and outputs of the mechanisms are also multiply realizable. A mousetrap is not medium independent; it must take a mouse as an input. A computer, though, is medium independent. A computer can be constructed from different parts assembled into different mechanisms and can take different types of inputs and outputs. In typical digital computers, the inputs and outputs are voltages, but in quantum computers, the inputs and outputs would be different.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Multiple realizability

Start with the simplest possible case. Write down what Multiple realizability claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Multiple realizability before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Multiple realizability ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Multiple realizability

In research: Multiple realizability appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Multiple realizability in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Multiple realizability is common in secondary-school and first-year university syllabi. It links to neighbouring topics Arguments in philosophy of mind, Cognitive science, Concepts in the philosophy of mind, so understanding it makes those chapters shorter.
In everyday life: Look for Multiple realizability outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Multiple realizability in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Multiple realizability means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Multiple realizability out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Multiple realizability in simple terms?

Why does Multiple realizability matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Multiple realizability?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Multiple realizability.