Out of Context: On Probability (Definitions)

"Probability is the very guide of life." (Marcus Tullius Cicero, "De Natura Deorum" ["On the Nature of the Gods"], 45 BC) [attributed] 

"Probability is a degree of possibility." (Gottfried W Leibniz, "On estimating the uncertain", 1676)

"Probability is the appearance of agreement upon fallible proofs." (John Locke, "An Essay Concerning Human Understanding", Book IV, 1689)

"Probability is likeliness to be true, the very notation of the word signifying such a proposition, for which there be arguments or proofs to make it pass, or be received for true." (John Locke, "An Essay Concerning Human Understanding", Book IV, 1689)

"Probability is a degree of certainty and it differs from certainty as a part from a whole." (Jacob Bernoulli, "Ars Conjectandi", 1713)

"[…] to us, probability is the very guide of life." (Joseph Butler, "The Analogy of Religion", 1736) 

"Probability is relative in part to this ignorance, and in part to our knowledge." (Pierre-Simon Laplace, "Mémoire sur les Approximations des Formules qui sont Fonctions de Très Grands Nombres", 1783) 

"Probability is expectation founded upon partial knowledge." (George Boole, "The Laws of Thought", 1854)

"Probability is, so far as measurement is concerned, closely analogous to similarity." (John M Keynes, "A Treatise on Probability", 1921)

"The Theory of Probability is concerned with that part which we obtain by argument, and it treats of the different degrees in which the results so obtained are conclusive or inconclusive." (John M Keynes, "A Treatise on Probability", 1921)

"Probability is the most important concept in modern science, especially as nobody has the slightest notion what it means." (Bertrand Russell, 1929)

"Probability is truth in some degree […]" (Errol E Harris, "Hypothesis and Perception: The Roots of Scientific Method", 1970)

"Probability, too, if regarded as something endowed with some kind of objective existence, is no less a misleading misconception, an illusory attempt to exteriorize or materialize our true probabilistic beliefs." (Bruno de Finetti, "Theory of Probability", 1974)

"The logic of certainty furnishes us with the range of possibility (and the possible has no gradations); probability is an additional notion that one applies within the range of possibility, thus giving rise to graduations (‘more or less’ probable) that are meaningless in the logic of uncertainty."  (Bruno de Finetti, "Theory of Probability", 1974)

"Many modern philosophers claim that probability is relation between an hypothesis and the evidence for it." (Ian Hacking, "The Emergence of Probability", 1975)

"The theory of probability is the only mathematical tool available to help map the unknown and the uncontrollable." (Benoit Mandelbrot, "The Fractal Geometry of Nature", 1977)

"Probability is the mathematics of uncertainty." (Richard W Hamming, "Methods of Mathematics Applied to Calculus, Probability, and Statistics", 1985)

"Probabilities are summaries of knowledge that is left behind when information is transferred to a higher level of abstraction." (Judea Pearl, Probabilistic Reasoning in Intelligent Systems: Network of Plausible, Inference, 1988)

"Probability is the branch of mathematics that describes randomness." (David S Moore, "Uncertainty", 1990)

"[...] probability is a style of thinking." (Richard W Hamming, "The Art of Probability for Scientists and Engineers", 1991)

"Probability is not about the odds, but about the belief in the existence of an alternative outcome, cause, or motive." (Nassim N Taleb, "Fooled by Randomness", 2001)

"Probability is a mathematical language for quantifying uncertainty." (Larry A Wasserman, "All of Statistics: A concise course in statistical inference", 2004)

"Probability is a liberal art; it is a child of skepticism, not a tool for people with calculators on their belts to satisfy their desire to produce fancy calculations and certainties." (Nassim N Taleb, “The Black Swan”, 2007)

"We have to be aware that probabilities are relative to a level of observation, and that what is most probable at one level is not necessarily so at another."(Carlos Gershenson, "Design and Control of Self-organizing Systems", 2007)

"Probability is the science of uncertainty. It provides precise mathematical rules for understanding and analyzing our own ignorance." (Michael J Evans & Jeffrey S Rosenthal, "Probability and Statistics: The Science of Uncertainty", 2009)