I've written the following brief history on how the definition of acids and bases change over time. Lavoisier came up with his definition circa 1776 in that 'p is acidic iff p contains much oxygen' and that held sway for 30 years until it was falsified by Davy in 1810 who demonstrated that the acid H2S lacked oxygen among others, though he offered no countertheory. Then in 1836 Liebig surmised that 'p is acidic iff p is a hydrogen-containing substance in which the hydrogen could be replaced by a metal' and this theory was accepted as orthodox for a good 50 years. In 1884 Arrhenius came up with 'p is acidic iff p increases the concentration of hydrogen ions (H+) in an aqueous solution' and 'p is basic iff p increases the concentration of hydroxide ions (OH-) in an aqueous solution. (6) In 1923 Bronstead and Lowry independently came up with the definition 'p is acidic and q is basic iff p donates a hydrogen ion (H+) to q.' (11) In 1938 Lewis posited that 'p is acidic and q is basic iff q donates an electron pair to p.' In the same year Usanovich hypothesized that 'p is acidic and q is basic iff q donates positive species to p.'(2) In 1947 Flood proposed that 'p is acidic and q is basic iff q donates an oxide ion (O2-) to p. I cannot figure out then when scientists propose a definition which is meant to put all acids and bases into a class because acids all share a common property what that property is. What is that property of acids which scientists agree they must have. For example, here is a chart of the Lewis' acids http://en.wikipedia.org/wiki/Lewis_acids_and_bases#/media/File:Lewis_Acids_and_Bases.jpg When you construct a definition there are certain beliefs you have about the class which you are not willing to give up. For example you come up with a list of 30 acids because they all have a certain property, then you try to figure out why they have that property by formulating that definition. Well, what is that property that acids have? The best I've been able to come up with is when an acid reacts with an alkali it forms a metal salt and water but of course that only applies to a special type of bases.  Miessler, G.L. Tarr, D.A., Inorganic Chemistry (1991) p 166.  Hall, Norris F. (March 1940). Systems of Acids and Bases. J. Chem. Educ. 17 (3) p 124-128  Meyers, R. (2003). The Basics of Chemistry. Greenwood Press. p 156.