Questions: Underdetermination of Theory by Evidence

Question 1

Two theories make identical predictions for every possible experiment — past, present, and future. A scientist argues that Theory A is preferable because it is mathematically simpler. What does the underdetermination thesis imply about this argument?

• A. The argument is invalid — if the theories make identical predictions, they are the same theory expressed differently
• B. The argument succeeds: simplicity is an empirical virtue, and simpler theories are better confirmed by the evidence
• C. The argument invokes a non-empirical criterion; underdetermination shows that evidence alone cannot settle the choice, so simplicity is doing epistemological work the evidence cannot do
• D. The argument fails because empirically equivalent theories are always both false

Answer: (C)

When theories are empirically equivalent, the choice between them genuinely cannot be made on purely empirical grounds — that is the core claim of underdetermination. The scientist's appeal to simplicity is an appeal to a theoretical virtue, not more evidence. The deeper debate — which the thesis opens up — is whether simplicity is truth-tracking (as realists argue) or merely a pragmatic preference (as empiricists argue). Underdetermination does not show the choice is arbitrary; it shows that making the choice requires going beyond the evidence.

Question 2

What are empirically equivalent theories?

• A. Theories that use the same mathematical formalism but different physical interpretations
• B. Theories that have been confirmed by the same set of past experiments
• C. Theories that make identical observational predictions for every possible experiment yet describe metaphysically different realities
• D. Theories from different scientific fields that describe the same phenomenon at different levels of analysis

Answer: (C)

Empirically equivalent theories are ones that agree on every observational prediction — not just the experiments done so far, but all possible future observations — yet disagree about the underlying nature of reality. Newton's mechanics with absolute space versus without a preferred rest frame is the classic case: no mechanical experiment can distinguish them, yet they describe different objective realities. This is more radical than merely being confirmed by the same data — it means no future data could discriminate between them either.

Question 3

Underdetermination implies that any scientific theory is just as well-supported as any competing alternative theory.

• T. True
• F. False

Answer: False

This is a common overstatement of the thesis. Underdetermination applies specifically to cases where theories are empirically equivalent — making identical predictions for all possible observations. Most competing theories are NOT empirically equivalent; they make different predictions that can in principle be tested. The thesis shows that in the specific (and philosophically significant) cases of empirical equivalence, evidence alone cannot decide — not that all theories are equally supported by evidence in general.

Question 4

The scientific realist and the empiricist differ over whether theoretical virtues like simplicity and fruitfulness are evidence of a theory's truth or merely reflect pragmatic cognitive preferences.

• T. True
• F. False

Answer: True

This is precisely the fault line the underdetermination thesis exposes. The realist argues: if simplicity is a reliable guide in science, it must be because simpler theories tend to be truer — the virtue is truth-tracking. The empiricist (or instrumentalist) counters: we prefer simple theories because they are easier to use and compute with, not because reality is itself simple; theoretical virtues select for useful tools, not true descriptions of unobservables. Underdetermination makes this dispute concrete and unavoidable.

Question 5

Why does the problem of underdetermination pose a fundamental challenge to scientific realism?

Model answer: Scientific realism holds that our best theories describe how reality actually is — including unobservable entities and processes. Underdetermination challenges this by showing that for any set of observations, multiple incompatible theories can account for them equally well. If the total evidence cannot uniquely determine which theory is true, the realist cannot claim that the theory we currently accept is the uniquely correct description of unobservable reality — it is just one of many empirically equivalent alternatives. This opens the door to instrumentalism: perhaps theories are just predictive tools, not true descriptions.

The threat is not to science's predictive success but to its metaphysical ambitions. A theory can be empirically adequate (save all the phenomena) without being uniquely true. Underdetermination makes it difficult to argue that theoretical virtues like simplicity are reliable guides to truth rather than convenience — and scientific realism needs those virtues to do epistemological work, because the evidence alone isn't enough.