 A proposition is in a sense formed by adding the name of the predicate to the name of the subject, so by adding ‘snow’ and ‘white’ we get ‘snow is white’. (We add ‘is’ as well, but as Hobbes argues, it’s not necessary, for we could indicate the same thing by word order rather than having an extra word as the copula.) In thinking about syllogisms, think about the example “Every man is an animal; every animal is a body; therefore every man is a body” (Hobbes 1655, 4.4). In some sense we add the propositions, or at least bits of them: we add the subject of the first proposition to the predicate of the second, aided in this by the middle term. 