Presuppositions are those background beliefs we all have that we don't really think about. Beliefs like, "My senses give me true information about the world," and "The future will resemble the past," are background beliefs that just about everybody has but that most people don't think about consciously. They just sort of automatically apply those beliefs to their experiences.
Sometimes our presuppositions are wrong. But whether they're right or wrong, they do have a big influence on how we filter information that comes our way. I was thinking about this last night and I came up with a real life example to explain what I mean.
Between the ages of 2 and 6, I lived in Abilene Texas. Out in west Texas, there aren't a whole lot of trees, but there is
a whole lot of sky. With all that sky, lightening storms are pretty amazing.
Let me back up a little. I remember watching on TV where this guy had a model of the earth and the sun. He was explaining night and day. On his little model, half the earth was covered in a black shell. He rotated the shell around the earth to explain night and day.
Now the shell obviously just represented the night sky, but you have to understand how something like that would look to a five year old kid. (I'm guessing I was about 5 at the time). I thought the model literally represented the way things are. I thought there really was half a shell around the earth that rotates around the earth. The light was all around, but the shell blocked it out on half the earth.
This belief became a given to me. It acted like a presupposition. So one night I was sitting in the drive way looking at the lightening spreading out across the sky. It was pretty amazing. As I watched it, I tried to understand it, and I remember my thinking. I was looking at the black sky thinking, "That's the black shell around half the earth." Whenever I'd see the lightening, I'd also hear the thunder, and I started to draw some conclusions. I figured what was going on was that this shell was under some pressure because of the storm. Every now and then it would crack because of the pressure. The cracks would let in the light from the other side, and that's what lightening was. It would also make a loud sound when the shell cracked, and that was the thunder.
Another night, I was looking at the stars and thinking about the shell. I figured that shell must be really old since it's been rotating around the earth forever. Since it's so old, it's a bit tattered and has some holes in it. The light comes in through the holes, and that's the stars.
See how that one presupposition influenced my interpretation of my observations? That's the way presuppositions are. So we ought to try to be more conscious of them. Bad presuppositions will result in us drawing the wrong conclusions about reality and prevent us from drawing the right conclusions.