Goodman's paradox

Riddle of induction (reasoning from the particular to the general) formulated by US philosopher Nelson Goodman. He invents a property ‘grue’, which applies to any green thing examined before a given time and also to any blue thing at any time, and uses it to show that in inductive reasoning some events do, and some do not, establish regularities from which we can make predictions, and that what determines our habits of classification is how deeply a property is entrenched in our thinking.

A prediction that all emeralds examined before the given time will be green, and a prediction that they will be ‘grue’, are both equally likely to be true. However, if, after the given time, we examine an emerald and it is ‘grue’, it must be blue and not green. Moreover, if the confirmation of predictions is defined in terms of past success, anything can be made to confirm anything else by inventing strange properties like ‘grue’. Some philosophers have criticized the device of a time-linked property as artificial.