What defines an infallible sign?

An infallible sign is defined as an effect that virtually always accompanies a certain cause, which directly ties the two together in a reliable manner. This concept is rooted in logic and reasoning, where an infallible sign serves as a definitive marker or indicator; if the cause is present, the effect will unavoidably occur.

For instance, in a scientific context, if a certain chemical reaction is known to always produce a specific gas, the presence of that gas can be seen as an infallible sign that the reaction has taken place. This reliability is what distinguishes infallible signs from other types of evidence or indicators, which may not consistently correlate with their associated causes.

Other options describe situations that do not meet the criteria for infallibility. Misconstrued effects, untestable assumptions, and effects with multiple interpretations lack the consistent, definitive relationship needed to characterize something as an infallible sign. Thus, the correct answer highlights the essence of infallibility in the connection between cause and effect.