Full Idea
Any modification that is unnatural (that is, forced) and sudden is painful, while any modification that restores the normal condition and is sudden is pleasant.
Gist of Idea
Unnatural modifications are painful, and restoring normality is pleasant
Source
Plato (Timaeus [c.362 BCE], 64d)
Book Reference
Plato: 'Timaeus and Critias', ed/tr. Waterfield,Robin [OUP 2008], p.62
A Reaction
[see also 65a] Possibly circular, if the painful is defined as unnatural, but the unnatural is defined as painful. Nowadays we find it very hard to specify what counts as 'unnatural', but our ancestors used that label all the time. Not convincing.