Most of the time when you hear Alfred Korzybski classic saying you do not hear the full version. Almost all of the time you get the abbreviation.
Basically it is a representational problem or a problem of perception. Things are not always what they seem to be.
A map is not the territory it represents, but if correct, it has a similar structure to the territory, which accounts for its usefulness.
You can use this as a tool when you get stuck in a process, a creation or something else. Remember that things are complex and we use simple system to explain these systems to ourselves.
For more information on this topic see the wikipedia.
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