The term counterfactual is short for "counter-to-fact conditional," a statement about what would have been true, had certain facts been different—for example, "Had the specimen been heated, it would have melted." On the face of it, claims about what would or could have happened appear speculative or even scientifically suspect because science is an investigation of reality grounded in experimental evidence, and by definition people have experimental access only to the actual universe. Yet, despite their implicit reference to alternative possibilities, many counterfactuals are scientifically respectable because the criteria determining whether they are true depend wholly on facts about the actual universe. Counterfactuals are often important in science because they appear implicitly in the definitions of certain specific concepts such as "solubility" and "biological fitness," and because they are close-ly related to general scientific notions such as "law of nature" and "causation."

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