Full Idea
It is not necessary for a sea-battle to take place tomorrow, nor for one not to take place tomorrow - though it is necessary for one to take place OR not take place tomorrow.
Gist of Idea
It is necessary that either a sea-fight occurs tomorrow or it doesn't, though neither option is in itself necessary
Source
Aristotle (On Interpretation [c.330 BCE], 19a30)
Book Reference
Aristotle: 'Categories and De Interpretatione', ed/tr. Ackrill,J.R. [OUP 1963], p.53
Related Idea
Idea 21389 Carneades distinguished logical from causal necessity, when talking of future events [Long on Carneades]