I found this one
We’ll perform a sort of theology that might be called “casting” or “molding”. We’re going to take something that we know is not like the Trinity, and which suffers serious inadequacies in representing it, but which nevertheless has a kernel of truth, and then we’ll try to knock away everything but that kernel of truth. When you cast a candle in a die, for example, you have a large iron mold that looks nothing like a candle, but which has within it some likeness to a candle. You use the die like a scaffolding that allows you to get to the point when you pull the die itself apart and are left with only the wax center that was within it. Here is what is going to serve as our die for understanding the Trinity: the action of using a paintbrush to make a line going from left to right. It’s a very simple idea but be sure the image is clear in your head. We’re talking about this… (Takes out paintbrush, makes a stroke from left to right.)
Now what am I doing when I do this? Notice that you can isolate three different things that happen:
1.) You desire to start at some point, say here (points at some location, sets the brush down there)
2.) You pull the brush left to right (do it).
3.) Then a third thing happens as a result, namely you not only get a line that goes left to right, it also has a top and a bottom. Now all you wanted to do was have a line that went left to right, but you find in doing this that you give rise to a top and a bottom too.
Now not everything in this image can be taken as illuminating something about the Trinity, but I want to draw your attention to the nature of this process. Note first that we can isolate a left-right relation which necessarily gives rise to a top-bottom relation. These relations are different and arise in a certain order. We want to draw the left before the right, and we get a result out of this that was outside our intention to have this horizontal distance. Nevertheless, while there is an order among the relations they are nevertheless all simultaneous: you cannot isolate a “pure left” or a “pure right”, nor can you keep this left right line from being a top that has a bottom. You have one undivided reality that admits of three distinct relative oppositions.
Now in this metaphor, “Left” is the Father, “right” is the Son. So what about the Holy Spirit? Notice that we can say that in one sense he proceeds from the Father-Son together (so far as they form a “top” to which he is “the bottom”) and in another sense proceeds from the Father alone, so far as everything that follows “the left” proceeds from it. Both expressions isolate some feature of the same reality, and so depending on what questions we want to answer, it might be more useful to consider the Holy Spirit as arising from the single Father-Son source or from the Father alone. It is also useful to describe the Holy Spirit as “from” the Father and “through” the Son, but this is simply a third account of the same reality. The accounts are not distinct from one another by one being true and the others false.
So much for the kernel of truth in the metaphor – where is it inadequate? What part is the “die” that needs to get pulled off and set aside? It is an incredibly difficult removal: we must isolate the relations themselves apart from the parts of the quantity that we know them in. Talking about “left-right” is impossible for us apart from some image of a physical body, but we have to separate out the pure relation from the physical parts of which it is the relation. Though we must look at a body to see a left and right, we have to tell ourselves that we are only considering the left right and not the body. But how can we understand this, given that we certainly can’t imagine it? It helps to return to our consideration of the intelligible difference between parts (or most accidents) and relations. The quantity has parts that each exist of themselves, but the relation consists precisely the co-existence of things. Quantitative parts, to the extent it has its own existence, must destroy the existence of the whole. You can’t break off or isolate a part of a quantity and still have one and the same whole exist. But relations are different: because they co-exist, it is possible to have a multiplication of terms that is not repugnant to an absolutely unified whole. This co-existence is not contrary to procession of one thing from another – in fact, it can even be based upon such a procession.