Justus von Liebig

"Chemistry, in its application to animals and vegetables. Endeavours jointly with physiology to enlighten us respecting the mysterious processes and sources of organic life."

"To investigate the essence of a natural phenomenon, three conditions are necessary: We must first study and know the phenomenon itself, from all sides; we must then determine in what relation it stands to other natural phenomena; and lastly, when we have ascertained all these relations, we have to solve the problem of measuring these relations and the laws of mutual dependence—that is, of expressing them in numbers. In the first period of chemistry, all the powers of men's minds were devoted to acquiring a knowledge of the properties of bodies; it was necessary to discover, observe, and ascertain their peculiarities. This is the alchemistical period. The second period embraces the determination of the mutual relations or connections of these properties; this is the period of phlogistic chemistry. In the third period, in which we now are, we ascertain by weight and measure and express in numbers the degree in which the properties of bodies are mutually dependent. The inductive sciences begin with the substance itself, then come just ideas, and lastly, mathematics are called in, and with the aid of numbers, complete the work."

"The acquisition of a new truth is like the acquisition of a new sense, which renders a man capable and recognizing a large number of phenomena that are hidden from another, as they were from him originally."

"Die Geschichte der Völker gibt uns Kunde von den ohnmächtigen Bemühungen der politischen und kirchlichen Gewalten, um Erhaltung des körperlichen und geistigen Sklaventhums der Menschen; die künftige Geschichte wird die Siege der Freiheit beschreiben, welche die Menschen durch die Erforschung des Grundes der Dinge und der Wahrheit errangen; Siege mit Waffen, an denen kein Blut klebt, und in einem Kampf, in welchem Moral und Religion sich nur als schwache Bundesgenossen betheiligten."

"Die Seife ist ein Massstab für den Wohlstand und die Cultur der Staaten."

"Ein jeder Aberglaube versetzt uns in das Heidenthum."

"Eine empirische Naturforschung in dem gewöhnlichen Sinn existiert gar nicht. Ein Experiment, dem nicht eine Theorie, d. h. eine Idee, vorhergeht, verhält sich zur Naturforschung wie das Rasseln mit einer Kinderklapper zur Musik."