Clarifying a Garrigou-Lagrange passage

[Baldrick]

Hi all 

I'm having a little trouble understanding what he is saying in this passage from Everlasting Life.  (A superb book btw! )

The concept of Being, of reality, underlies all other concepts.  The verb "to be" underlies every thought and sentence.  "Peter runs" means "peter is running".

So far, so good.  Then he says: 

In i priori judgements this "is" expresses essence.

...hmmm...meaning, that a priori judgements express what a thing is.  So, the judgement "man [subject] is a mortal being [predicate]" expresses the nature/essence of man.

So then he says: 

In a posteriori judgements the "is" expresses existence.

....meaning, that it expresses that that a thing is, I suppose. 

Can someone help elucidate this for me? 

[An aspiring Thomist]

"In i priori judgements this "is" expresses essence."I think in the above case the "is" expresses what a thing is in its essence wether or not it exists in act or only in potency.

In the other case the "is" expresses that a thing exists in actuality.

I am currently reading Providence by Fr. Lagrange. Very good read also.

[John Lamb]

When I say "a triangle is a three-sided polygon", the is tells us the essence of a triangle, what it means to be triangle irrespective of existence (the statement doesn't affirm the existence of any triangle in particular).
When I point to a sign and say "that sign is a triangle", the is is talking about the existence of a particular triangle, affirming its existence irrespective of its essence (the statement doesn't define the essence of a triangle, only that a particular existing object is one).

Fr. Garrigou-Lagrange is saying that there are two meaning of "is" or "to be", whether relating to its essence (a priori) or its existence (a posteriori). Like Thomist says, in the a posteriori case we are looking at what exists in the world, whereas in the a priori case we are looking at the essence or nature of things whether or not they exist in the world.