William Thomson

Do not imagine that mathematics is hard and crabbed, and repulsive to common sense. It is merely the etherealization of common sense.

...when you can measure what you are speaking about, and express it in numbers, you know something about it;...

William Thomson (June 26, 1824–December 17, 1907), 1st Baron Kelvin, often referred to simply as Lord Kelvin, was a Scottish physicist.

Sourced [edit]

I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.
- Lecture on "Electrical Units of Measurement" (3 May 1883), published in Popular Lectures Vol. I, p. 73; quoted in Encyclopaedia of Occupational Health and Safety (1998) by Jeanne Mager Stellman, p. 1992

There cannot be a greater mistake than that of looking superciliously upon the practical applications of science. The life and soul of science is its practical application; and just as the great advances in mathematics have been made through the desire of discovering the solution of problems which were of a highly practical kind in mathematical science, so in physical science many of the greatest advances that have been made from the beginning of the world to the present time have been made in earnest desire to turn the knowledge of the properties of matter to some purpose useful to mankind.
- Lecture on "Electrical Units of Measurement" (3 May 1883), published in Popular Lectures Vol. I, p. 73, as quoted in The Life of Lord Kelvin (1910) by Silvanus Phillips Thompson

Quaternions came from Hamilton after his really good work had been done, and though beautifully ingenious, have been an unmixed evil to those who have touched them in any way.
- Letter to R. B. Hayward (1892), as quoted in Energy and Empire : A Biographical Study of Lord Kelvin (1989) by Crosbie Smith and M. Norton Wise

I am afraid I am not in the flight for “aerial navigation”. I was greatly interested in your work with kites; but I have not the smallest molecule of faith in aerial navigation other than ballooning or of expectation of good results from any of the trials we hear of. So you will understand that I would not care to be a member of the aëronautical Society.
- As a response to Major B. F. S. Baden Powell's request to join the Aeronautical Society, December 8, 1896.
- Often reproduced out of context and without citation to any primary source as "Heavier-than-air flying machines are impossible", like in The Experts Speak : The Definitive Compendium of Authoritative Misinformation (1984) by Christopher Cerf and Victor Navasky, p. 236

Symmetrical equations are good in their place, but 'vector' is a useless survival, or offshoot from quaternions, and has never been of the slightest use to any creature.
- Letter to G. F. FitzGerald (1896) as quoted in A History of Vector Analysis : The Evolution of the Idea of a Vectorial System (1994) by Michael J. Crowe, p. 120

Do not imagine that mathematics is hard and crabbed, and repulsive to common sense. It is merely the etherealization of common sense.
- Quoted in Life of Lord Kelvin (1910) by Silvanus Phillips Thompson

Tesla has contributed more to electrical science than any man up to his time.
- Statement of 1896, as quoted in Prodigal Genius : The Life of Nikola Tesla (2007) by James J. O'Neill

To live among friends is the primary essential of happiness.
- Lord Kelvin’s Replies to Addresses given on the Celebration of the Jubilee of his Professorship (June 15-17, 1896). Quoted in Lord Kelvin, Professor of Natural Philosophy in the University of Glasgow 1846-1899 by George F. Fitzgerald, 1899. Full Text

There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.
- Although reportedly from an address to the British Association for the Advancement of Science (1900), the quote is only duplicated without citation to any primary source in various books, including Superstring : A theory of everything? (1988) by Paul Davies and Julian Brown; also in Rebuilding the Matrix : Science and Faith in the 21st Century (2003) by Denis Alexander. To be more credible, a source prior to the 1980s and close to 1900 is needed.
  - Confusion may be due to Michelson who made a similar quote whilst mentioning Lord Kelvin: In 1894, Albert A. Michelson remarked that in physics there were no more fundamental discoveries to be made. Quoting Lord Kelvin, he continued, “An eminent physicist remarked that the future truths of physical science are to be looked for in the sixth place of decimals. [1]

The beauty and clearness of the dynamical theory, which asserts heat and light to be modes of motion, is at present obscured by two clouds. I. The first came into existence with the undulatory theory of light, and was dealt with by Fresnel and Dr. Thomas Young; it involved the question, how could the earth move through an elastic solid, such as essentially is the luminiferous ether? II. The second is the Maxwell–Boltzmann doctrine regarding the partition of energy.
- From a 1900, April 27, Royal Institution lecture. Lord Kelvin, Nineteenth Century Clouds over the Dynamical Theory of Heat and Light, Philosophical Magazine, Sixth Series, 2, 1–40 (1901). Science 100 Years Ago two clouds

Wikipedia has an article about:

William Thomson, 1st Baron Kelvin

Wikisource has original works written by or about:

William Thomson

Wikimedia Commons has media related to:

William Thomson