Wisdom from the Early Stoics, Zeno of Citium 46

A probable judgement is one which induces to assent, e.g. "Whoever gave birth to anything, is that thing's mother." This, however, is not necessarily true; for the hen is not mother of an egg. 

Again, some things are possible, others impossible; and some things are necessary, others are not necessary. 

A proposition is possible which admits of being true, there being nothing in external circumstances to prevent it being true, e.g. "Diocles is alive." 

Impossible is one which does not admit of being true, as e.g. "The earth flies." 

That is necessary which besides being true does not admit of being false or, while it may admit of being false, is prevented from being false by circumstances external to itself, as "Virtue is beneficial." 

Not necessary is that which, while true, yet is capable of being false if there are no external conditions to prevent, e.g. "Dion is walking." 

A reasonable proposition is one which has to start with more chances of being true than not, e.g. "I shall be alive tomorrow."

—Diogenes Laërtius, 7.75-76