Validity is the truth of an inference made from evidence and is a central concern in scientific work. Given the maturity of the domain of music information research (MIR), validity in our opinion should be discussed and considered much more than it has been so far. Puzzling MIR phenomena like adversarial attacks, horses, and performance glass ceilings become less mysterious through the lens of validity. In this paper, we review the subject of validity as presented in a key reference of causal inference: Shadish et al., "Experimental and Quasi-experimental Designs for Generalised Causal Inference". We discuss the four types of validity and threats to each one. We consider them in relationship to MIR experiments grounded with a practical demonstration using a typical MIR experiment.