Paula agreed to mow John's yard once a week for $50 per week throughout the summer. Paula, however, was having trouble getting her money from John. On one occasion, he gave her a handwritten IOU stating "I, John Jones, owe Paula Smith $50" which he signed at the end. A couple of weeks later, John did not have the money with which to pay Paula for additional mowing, and he handwrote the following on a piece of paper and gave it to her: "I, John Jones, promise to pay Paula Smith or to bearer, the sum of $100 on Monday, July 22, 2017." Paula quit mowing John's yard, and disgusted with John, Paula assigned both documents to Vince. When Vince presented the documents to John, John refused to pay on the basis that after inspecting the yard, he decided that Paula was doing a poor job. Vince told him the documents constituted negotiable instruments, but John disagreed.

Is the handwritten statement "I, John Jones, owe Paula Smith $50" coupled with the signature of John Jones sufficient to constitute a negotiable instrument? Explain.