How do we translate this sentence into QL? The sentence is about some ravens, and about some black things. Thus we might be tempted to try:

where "Rx" is "x is a raven: and "Bx" is "x is black. The problem with this is that it is telling us that some things are ravens and some things are black. But those things might not be the same thing. What we're really after is a way of saying that there is something that is both a raven and a black thing.  To express this we need the following symbolization:

We read this as follows: "There is something which is both a raven and black." Notice that instead of having two existentially quantified propositions conjoined, we have  an existential quantifier ranging over the conjunction. This represents a new class of proposition, one we haven't encountered until now. We call propositions like this categorical propositions. 

The logic of categorical propositions, (which are central to Quantificational Logic) was fairly well understood in ancient times. We're going to step back and look at the (mostly) true things ancient logicans had to say about such propositions before we return to representing them in QL.

Before going on, you should complete the following Mixed Translation Exercise, which contains a mixture of the different types of quantified propositions we've studied so far.