Wittgenstein, On Certainty, 338:
But imagine people who were never quite certain of these things, but said they were very probably so, and that it did not pay to doubt them. Such a person, then, would say … “It is extremely unlikely that I have ever been on the Moon,” etc, etc. How would the life of these people differ from ours? For there are people who say that it is merely extremely probable that water over a fire will boil and not freeze, and that therefore strictly speaking what we consider impossible is only improbable. What difference does it make in their lives? Isn’t it just that they talk rather more about certain things than the rest of us?