Full Idea
Logicians have come increasingly to realise that logic is the science not of the intension, but of the extension of terms.
Clarification
'Intension' is the meaning of a word; 'extension' is the things it refers to
Gist of Idea
Nowadays logic is seen as the science of extensions, not intensions
Source
Roger Scruton (Short History of Modern Philosophy [1981], Ch.4)
Book Reference
Scruton,Roger: 'A Short History of Modern Philosophy' [ARK 1985], p.45
A Reaction
I take this to be because the notion of a 'set' is basic, which is defined strictly in terms of its members. This move is probably because we can be clear about extensions, but not intensions. Tidiness is no substitute for complex truth.
Related Idea
Idea 9457 The two main views in philosophy of logic are extensionalism and intensionalism [Jacquette]