A conclusion based on silence or lack of contrary evidence.
John: Do you know any Spanish? Jack: Of course. I speak it like a native. John: That's good, because I need to know the Spanish phrase for "Happy Birthday". Jack: Sorry, I don't have time for that right now. Maybe tomorrow. Bye.
Afterwards, Jack continually refuses to give John the Spanish translation, either by ignoring John or by giving excuses. John then concludes, by argument from silence, that Jack does not in fact know Spanish or does not know it well. In other words, John believes that Jack's ignorance is the most plausible explanation for his silence. Use of argument from silence in this situation is reasonable given the alternatives, that Jack either doesn't want or is afraid to translate, would be unreasonable without more information.
Explore reasons why the subject would not answer the question. There may be a good reason and this could invalidate the conclusion from lack of evidence.