[Before college] I was almost more interested in literature and history than in the exact sciences; I was equally good in all subjects including the classical languages.

After the discovery of spectral analysis no one trained in physics could doubt the problem of the atom would be solved when physicists had learned to understand the language of spectra. So manifold was the enormous amount of material that has been accumulated in sixty years of spectroscopic research that it seemed at first beyond the possibility of disentanglement. An almost greater enlightenment has resulted from the seven years of Röntgen spectroscopy, inasmuch as it has attacked the problem of the atom at its very root, and illuminates the interior. What we are nowadays hearing of the language of spectra is a true 'music of the spheres' in order and harmony that becomes ever more perfect in spite of the manifold variety. The theory of spectral lines will bear the name of Bohr for all time. But yet another name will be permanently associated with it, that of Planck. All integral laws of spectral lines and of atomic theory spring originally from the quantum theory. It is the mysterious organon on which Nature plays her music of the spectra, and according to the rhythm of which she regulates the structure of the atoms and nuclei.

Although my Aachen colleagues and students at first regarded the “pure mathematician” with suspicion, I soon had the satisfaction of being accepted a useful member not merely in teaching but also engineering practice; thus I was requested to render expert opinions and to participate in the Ingenieurverein [engineering association].

Consciously and systematically Klein sought to enthrall me with the problems of mathematical physics, and to win me over to his conception of these problems as developed it in lecture courses in previous years. I have always regarded Klein as my real teacher only in things mathematical, but also in mathematical physics and in my conception of mechanics.

If I knew something about it, I wouldn’t lecture on it!

If you want to be a physicist, you must do three things—first, study mathematics, second, study more mathematics, and third, do the same.

Mathematics is like childhood diseases. The younger you get it, the better.

Moreover I can assure you that the misuse word “national” by our rulers has thoroughly broken me of the habit of national feeling that was pronounced in my case. I would now be willing see Germany disappear as a power and merge into a pacified Europe.

My father, the practicing physician, … was a passionate collector of natural objects (amber, shells, minerals, beetles, etc.) and a great friend of the natural sciences. … To my energetic and intellectually vigorous mother I owe an infinite debt.

The same applies to the concept of force as does to any other physical concept: Verbal definitions are meaningless; real definitions are given through a measuring process.

Thermodynamics is a funny subject. The first time you go through it, you don’t understand it at all The second time you go through it, you think you understand it, except for one or two points. The third time you go through it, you know you don't understand it, but by that time you are so used to the subject, it doesn't bother you anymore.

What I especially admire about you [Arnold Sommerfeld] is the way, at a stamp of your foot, a great number of talented young theorists spring up out of the ground.