We have only what we know, what we have observed, what we have experienced, to reflect in our dreams. It is impossible for the mind to conceive of something that does not exist. Over time I have discovered that my dream comes to me more readily after ingesting certain meals. A large, unusual meal of any kind will bring on a dream. I think the quantity has more effect than the quality.
Legumes seem to be a big dream maker.
If you will bear with me, I will proceed with an experiment. I will take a short break from this writing chore to attempt producing my flying dream again. I will try to write down my impressions as soon as I awake. The most exciting dream of flying I have ever had. You won't believe what I am about to relay.