Full Idea
There is a popular view in Megara, that x is capable of being/doing the F only when it actually is/does the F. So the non-builder is no bearer of a potentiality for building - but only when the builder is engaged in his building.
Gist of Idea
The Megarans say something is only capable of something when it is actually doing it
Source
Aristotle (Metaphysics [c.324 BCE], 1046b28)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.258
A Reaction
This Megaran view is the extreme denial of dispositions are real features of the world. They seem to reduce to mere descriptions, when the reality is the actual activity itself. Megarans would now be called 'actualists'.