Legal Principle: The act of using influence on another and taking undue advantage of that person is called undue influence. Factual Situation: A, an illiterate old man, was made to put his signature on a document which was in fact a pro-note. A thought the document to be a will, where his signature was required as a witness. Under this presumption, he signed the document which was in fact a pro-note. Subsequently, B endorsed the pro-note to C who paid value for it in good faith. C sued A on the pro-note. Issue: Is A liable? Decision: