Dekalog: One (Polish: Dekalog, jeden) is the first part of Dekalog, the drama series of films directed by Polish director Krzysztof Kieślowski for television, possibly connected to the first and second imperatives of the Ten Commandments: "I am the Lord your God; you shall have no other gods before me" and "Thou shalt not make unto thee any graven image."

A university professor (Henryk Baranowski) trains his young son in the use of reason and the scientific method, but is confronted with the unpredictability of fate. Reason is defied with tragic results.