Wisdom from the Early Stoics, Zeno of Citium 21

The study of syllogisms they declare to be of the greatest service, as showing us what is capable of yielding demonstration; and this contributes much to the formation of correct judgements, and their arrangement and retention in memory give a scientific character to our conception of things.

An argument is in itself a whole containing premises and conclusion, and an inference (or syllogism) is an inferential argument composed of these. 

Demonstration is an argument inferring by means of what is better apprehended to something less clearly apprehended.

A presentation (or mental impression) is an imprint on the soul: the name having been appropriately borrowed from the imprint made by the seal upon the wax.  

There are two species of presentation, the one apprehending a real object, the other not. 

The former, which they take to be the test of reality, is defined as that which proceeds from a real object, agrees with that object itself, and has been imprinted seal-fashion and stamped upon the mind

The latter, or non-apprehending, that which does not proceed from any real object, or, if it does, fails to agree with the reality itself, not being clear or distinct.  

—Diogenes Laërtius, 7.45-46