think of your mind
if you understand your mind you will realize that it is made up of different functional parts. a thought cannot exist but in response to something. in the human mind, thoughts are generated by friction or you could say by a sort of "dialogue" between different parts. a thought is either a question rising from stimulus (internal or external) or it is an answer to a question posed by another thinking entity (internal or external).
we've heard of the id, ego, and superego. whether or not these are completely accurate description of the mind's anatomy is besides the point, though. it is clear that there are different competing parts in our mind, and that is what makes up the character of our mind. if an individual component of the mind was completely self-sufficient, there would be no need for the other parts. yet there are other parts, and they are necessary to the functioning of the whole.
the way i understand the concept of the trinity is very much how i understand the human mind. you may use the word "i" or "me" to designate the "self" as a whole. yes, the self can be sub-divided into parts, but none of these parts can represent the whole. the holy trinity--the father, the son, and the holy ghost--are parts of a whole. jesus isnt the whole of god. the father isnt the whole of god. the holy spirit isnt the whole of god. yet they are all god. they are different functional aspects of the whole.
the holy spirit is the most mysterious of the components because it is the one part that has not been personified for us. it exists as some necessary counterpoint to the father and son, but it isnt clear exactly what it is. i like the idea that the holy ghost is the mother (and mary is sort of a surrogate). there was a lot of misogyny in biblical times, and perhaps the idea of a holy mother wasn't popular, so they made her into a "ghost" or "spirit" instead. you need some sort of mother figure as part of the godhead, because a "father" cannot create by itself. nor can a mother. they are necessary counterpoints in the creation of things. and the son is obviously the product and union of these two components, yet not sufficient on its own (a son must be created--it cannot create itself--therefore, jesus cannot be seen as a "whole" god). this little system is open-ended (to continue, where is the "son's" wife and child? they are not part of the god-head, clearly, yet conceptually they must exist in some form. i believe any godhead must be open-ended. if it were truly self-contained, then whence reality?)