Full Idea
Among terms which appear to exist, there are, we may say, four great classes: 1) instants, 2) points, 3) terms which occupy instants but not points, 4) terms which occupy both points and instants. Analysis cannot explain 'occupy'.
Gist of Idea
Four classes of terms: instants, points, terms at instants only, and terms at instants and points
Source
Bertrand Russell (The Principles of Mathematics [1903], §437)
Book Reference
Russell,Bertrand: 'Principles of Mathematics' [Routledge 1992], p.465
A Reaction
This is a massively reductive scientific approach to categorising existence. Note that it homes in on 'terms', which seems a rather linguistic approach, although Russell is cautious about such things.