Difficult things to pin down. As described in the RugBook, "centers" are, according to ChristopherAlexander, the fundamental structures from which all aesthetically pleasing patterns are built.
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Centers are things you notice.  They are things you might give names to.
They are things we perceive as structures.  So, houses are centers, as
are doors, doorknobs, and a circle on the end of a doorknob.  Big centers
are made up of little centers.

Oh - like objects? -- OleAndersen

If I am building a simulation of an exploding star in Fortran, none of the centers
of the simulation are objects, because the system is not object-oriented.  So, I take the
question to mean "In an object-oriented system, are centers and objects the same thing?"

Maybe centers are objects, but I don't think objects are necessarily centers.  
In general, if I find that something is worth making a name for, I find that
it is worth defining an object for it.  But there are certainly objects that are not centers.
In most object-oriented systems, some of the objects are needed
for implementation purposes but are really not important to think about most of the 
time. -RalphJohnson

''Curious, I always felt that the interfaces were more interesting - between inside and outside, between land and sea, between this and that; the centres were just homogenous fillings-in and things only distinguished themselves by how they set themselves apart from their surroundings.''

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CategoryJargon