A researcher claims that all human dreams are driven by repressed childhood desires. If a patient dreams of a happy childhood memory, the researcher claims this represents a fulfilled wish. If the patient dreams of a terrifying falling event, the researcher claims this is a 'disguised' wish-fulfilment protecting the patient from waking up.

Because any possible dream outcome can be interpreted to support this theory, meaning no empirical evidence can ever disprove it, which key feature of science does this theory lack?