He says this because it is a known fact that human testimony can be either be true or false. Consenquently, it is important for people to understand the difference between a reliable and an unreliable source of information. Hume's answer would be that the more probable is also the most reliable. When looking at the legitimacy of a miracle, the question becomes whether the laws of nature are more probable, therefore reliable, then the word of the witness. Hume could conclude that the laws of nature will always be more reliable then the word of a human. His reasoning being that the laws of nature have never been wrong and the words of humans have been. For example, it is improbable for someone to be capable of parting the sea, a supposed miracle, because that defies the laws of nature. The greater probability would that the witness was giving false information and noone has ever parted the sea. This means that in order for a miracle to be considered valid, it must be more probable to believe the witness to the miracle then to believe ones personal past experiences. Because this will never be the case, Hume can declare miracles to be impossible. Hume's conclusion comes from the following thought process: beliefs about matter of fact are determined according to probability, laws of nature always have the most probability, anything that opposes the laws of nature is not worth believing, and finally all miracles defy the law of nature. Each step of the thought process leads Hume to the final conclusion that miracles are not worth believing in and are the least probable thing to occur.
Further into section X, Hume says why it is that people continue to believe in miracles even though miracles are obviously improbable. His first reason concerns human feelings. It's Hume's belief that people get a gratifying feeling when they believe in a miracle. The second reason ties miracles to religion.
