NLP presupposes that there isn't an absolute version of reality! But how can this be?
Well, everyone creates their own version of reality which is based upon their life experiences and the current thinking they have at that time.
The most obvious example which explains this most easily is Father Christmas. When children are young they have an absolute belief that Father Christmas is real, but as they get older they will re-create another reality in which he doesn't exist anymore. So what this example confirms, is that our current beliefs can be very different to what we belived in our past. Not only our past from a long time ago as a child, but quite often from our very recent past.
Due to this nature of reality, NLP uses an expression which is "the map is not the territory". Which is developed from a slightly lengthier original by Korzybski in "Science and Sanity".
What this is saying in simple terms is that the territory is the actual objective reality. The map is our percieved subjective reality.
The five senses that we use to interact with the world around us will vary from person to person and greatly between species. I know that my hearing is better than my girlfriends hearing and I can hear a higher range of sounds from a greater distance, yet my dog can hear way beyond my spectrum of hearing. The same with eyes and sense of smell, which means that our map that we build of what is actually out their (the territory) will be based only on the data that we have collected, not on the full set of data that exists out there which other people and animals may experience.
Because our brains limit the amount of information we can take in at any one time we can never get a complete and accurate picture of the territory. We also have filters that look for patterns which match our beliefs, values, language and other things we have experienced. Things that don't match are deleted, distorted or generalised.
So this filtered information is used as a re-presentation of a percieved reality and this is unique to the person and this becomes their map. Just like a real map, each individuals own map only has some of the features of the territory. Which means it is only good for certain things and just like a real map it can become out of date quickly if it isn't updated.
Summary of Reality
So perceptions of the world we live in are collected using our sensory input and this is compared against previously stored experiences. Once they are compared we then create a meaning which we then use to make decisions.
So our behaviour is just a reflection of these decisions we made and the meaning that we created. And because this process is a personal construction which makes our reality self-constructed, we all have a different idea of what reality is.