Influenced by Pierre Duhem's argument that hypotheses cannot be tested in isolation from the whole theoretical network in which they figure, Quine thinks that an hypothesis is verified true or false only relative to background assumptions. There is no certain way to determine what has to be changed in a theory, any hypothesis can be retained as true or discarded as false by performing appropriate adjustments in the overall network of assumptions. No sentence has special epistemic properties that safeguard it from revision.
Science is but self-conscious common sense.
The child's concepts are based on the notion of similarity, and they slowly evolve to acquire a more theoretical structure. The truth of a statement cannot be assessed as a function of the meaning of its words. Words do not have an absolute meaning. They have a meaning only with respect to the other words they are connected to in the sentences that we assume to be true. Their meaning can even change in time.