Since we hope to learn something about science and its operation and since science concerns itself with a certain type of knowledge and its attainment, let us begin with a brief consideration of how we arrive at knowledge. A common type of knowledge is that based on opinion or on the acceptance of another's authority. Most of our everyday knowledge used for dealing with the practicalities and necessities of life is gained in either of these fashions. Much of what one learns in the course of reading a book is taken as true simply because it has been presented on the printed page, although hopefully you will be more critical than that. Thus, we can merely have an opinion about a proposition and then decide to accept it as true or we can appeal to the authority of another, as ‘Einstein says’ or ‘Aristotle says,’ or to the authority of a text, as ‘The Bible says,’ or we can assert a fact to be ‘obvious’. Consider the following example: