Why are certain things doubtful and not doubtful?

(Thus is not improper to conclude from this that phisics, astronomy, medicine, and all other desciplines that are dependant upon the consideration of composite things that are doubtful, and that, on the other hand, arithmetic, geometry, and other such disciplines, which treat of nothing but the simplest and most general things and which are indifferent as to wether these things do or do not in fact exist, contain something certain or indubitable.) the Meditator concludes, though he can doubt composite things like medicine, phisics and astronomy. The meditator cannot doubt the simple and universal parts from which they are constructed like shape, quantity, size, (arithmetic and geometry). The Meditator says weather his sleep or awake you can't doubt the arithmetic or geometry. It can't be possible that such facts can be a dream. In this part the meditator is trying to prove that not all is a dream, there are things that are real and that you cannot be decieve and be doubtful. That is probably why the meditator puts his dreams as dreams and than says certains things are not made to be dreams. This part is very confusing, many readers probably are confuse just like me.