159. As children we learn facts; e.g., that every human being has a brain, and we take them on trust. I believe that there is an island, Australia, of such-and–such a shape, and so on and so on….
160. The child learns by believing the adult. Doubt comes after belief.
161. I learned an enormous amount and accepted it on human authority, and then I found some things confirmed or disconfirmed by my own experience.
162. In general I take as true what is found in text-books, of geography for example. Why? I say: All these facts have been confirmed a hundred times over. But how do I know that? What is my evidence for it? I have a world-picture. Is it true or false? Above all it is the substratum of all my enquiring and asserting. The propositions describing it are not all equally subject to testing.
163. … [W]henever we test anything, we are already presupposing something that is not tested….
164. Doesn’t testing come to an end?
165. One child might say to another: “I know that the earth is already hundreds of years old” and that would mean: I have learnt it.
166. The difficulty is to realize the groundlessness of our believing.
Ludwig Wittgenstein, On Certainty pp. 23-4